[Cisco Specific] Fix to verify ecn marking when egress port is on long link (HBM) card (sonic-net#17668)

Description of PR
for cisco-8000 chassis in packet mode, add a test to verify ecn marking at congestion points based on traffic pattern

Summary:
Fixes # (issue)

When multiple backplane traffic flows share the same slice of a asic on long-link card and use the default VOQ (as HBM-based long-link cards operate in default VOQ mode), the XOFF threshold gets breached.

This results in an XOFF signal being generated to the Fabric port, which leads to ECN marking at the Fabric port.

However, since no actual congestion is experienced on the egress port, it does not trigger ECN marking, leading to the test case failure.

Approach
What is the motivation for this PR?
How did you do it?
if ecn counter is not zero at egress port->success

if ecn counter is zero:

check the backplane ports on all FC - some of them should have non-zero ecn counters. pass the test else step 2

check the backplane ports on the ingress port LC — some of them should have non-zero ecn counters. pass the test else step 3

If the two checks above led to all zero ecn counter -

Check that the ingress ports did generate XOFF else fail the test case

How did you verify/test it?
Verify correctness by checking that ecn marking occurs before signaling XOFF

Any platform specific information?
Cisco T2

-- Docs: https://docs.pytest.org/en/stable/how-to/capture-warnings.html
============================================================================================================================================ PASSES =============================================================================================================================================
_______________________________________________________________________________________________________________________ test_ecn_marking_port_toggle[multidut_port_info0] _______________________________________________________________________________________________________________________
_______________________________________________________________________________________________________________________ test_ecn_marking_port_toggle[multidut_port_info1] _______________________________________________________________________________________________________________________
_______________________________________________________________________________________________________________________ test_ecn_marking_port_toggle[multidut_port_info2] _______________________________________________________________________________________________________________________
_______________________________________________________________________________________________________________________ test_ecn_marking_port_toggle[multidut_port_info3] _______________________________________________________________________________________________________________________
____________________________________________________________________________________________________________ test_ecn_marking_lossless_prio[multidut_port_info1-test_flow_percent0] _____________________________________________________________________________________________________________
____________________________________________________________________________________________________________ test_ecn_marking_lossless_prio[multidut_port_info1-test_flow_percent1] _____________________________________________________________________________________________________________
____________________________________________________________________________________________________________ test_ecn_marking_lossless_prio[multidut_port_info1-test_flow_percent2] _____________________________________________________________________________________________________________
____________________________________________________________________________________________________________ test_ecn_marking_lossless_prio[multidut_port_info1-test_flow_percent3] _____________________________________________________________________________________________________________
____________________________________________________________________________________________________________ test_ecn_marking_lossless_prio[multidut_port_info1-test_flow_percent4] _____________________________________________________________________________________________________________
____________________________________________________________________________________________________________ test_ecn_marking_lossless_prio[multidut_port_info1-test_flow_percent5] _____________________________________________________________________________________________________________
____________________________________________________________________________________________________________ test_ecn_marking_lossless_prio[multidut_port_info1-test_flow_percent6] _____________________________________________________________________________________________________________
____________________________________________________________________________________________________________ test_ecn_marking_lossless_prio[multidut_port_info2-test_flow_percent0] _____________________________________________________________________________________________________________
____________________________________________________________________________________________________________ test_ecn_marking_lossless_prio[multidut_port_info2-test_flow_percent1] _____________________________________________________________________________________________________________
____________________________________________________________________________________________________________ test_ecn_marking_lossless_prio[multidut_port_info2-test_flow_percent2] _____________________________________________________________________________________________________________
____________________________________________________________________________________________________________ test_ecn_marking_lossless_prio[multidut_port_info2-test_flow_percent3] _____________________________________________________________________________________________________________
____________________________________________________________________________________________________________ test_ecn_marking_lossless_prio[multidut_port_info2-test_flow_percent4] _____________________________________________________________________________________________________________
____________________________________________________________________________________________________________ test_ecn_marking_lossless_prio[multidut_port_info2-test_flow_percent5] _____________________________________________________________________________________________________________
____________________________________________________________________________________________________________ test_ecn_marking_lossless_prio[multidut_port_info2-test_flow_percent6] _____________________________________________________________________________________________________________
____________________________________________________________________________________________________________ test_ecn_marking_lossless_prio[multidut_port_info2-test_flow_percent7] _____________________________________________________________________________________________________________
_____________________________________________________________________________________________________________________ test_ecn_marking_ect_marked_pkts[multidut_port_info0] _____________________________________________________________________________________________________________________
_____________________________________________________________________________________________________________________ test_ecn_marking_ect_marked_pkts[multidut_port_info1] _____________________________________________________________________________________________________________________
_____________________________________________________________________________________________________________________ test_ecn_marking_ect_marked_pkts[multidut_port_info2] _____________________________________________________________________________________________________________________
_____________________________________________________________________________________________________________________ test_ecn_marking_ect_marked_pkts[multidut_port_info3] _____________________________________________________________________________________________________________________
-------------------------------------------------------------------------------------------------------------- generated xml file: /run_logs/ixia/21831/2025-03-25-11-57-36/tr.xml --------------------------------------------------------------------------------------------------------------
INFO:root:Can not get Allure report URL. Please check logs

co-authorized by: jianquanye@microsoft.com

Author: sreejithsreekumaran