Move bgp/test_bgp_stress_link_flap.py to onboarding test job (sonic-net#14145)

What is the motivation for this PR?
bgp/test_bgp_stress_link_flap.py is a newly added test script, but it have chance to get stuck for unknown reason:
https://elastictest.org/scheduler/testplan/66beb14c279305ddb0527383?leftSideViewMode=detail&prop=end_time&order=descending&testcase=bgp%2Ftest_bgp_stress_link_flap.py&type=console

How did you do it?
Move to onboarding test job, if stable, in 1 week, will move back to PR test set

How did you verify/test it?

Co-authored-by: xwjiang2021 <96218837+xwjiang2021@users.noreply.github.com>

Author: xwjiang-ms

.azure-pipelines/pr_test_scripts.yaml

@@ -20,7 +20,6 @@ t0:
   - bgp/test_bgp_speaker.py
   - bgp/test_bgp_update_timer.py
   - bgp/test_bgpmon.py
-  - bgp/test_bgp_stress_link_flap.py
   - bgp/test_bgp_session.py
   - cacl/test_cacl_application.py
   - cacl/test_cacl_function.py
@@ -254,7 +253,6 @@ t1-lag:
   - bgp/test_bgp_multipath_relax.py
   - bgp/test_bgp_update_timer.py
   - bgp/test_bgpmon.py
-  - bgp/test_bgp_stress_link_flap.py
   - bgp/test_bgp_session.py
   - bgp/test_traffic_shift.py
   - configlet/test_add_rack.py
@@ -445,6 +443,7 @@ dpu:
   - dash/test_dash_vnet.py
 
 onboarding_t0:
+  - bgp/test_bgp_stress_link_flap.py
   - copp/test_copp.py
   - crm/test_crm.py
   # We will add a batch of T0 control plane cases and fix the failed cases later
@@ -459,6 +458,7 @@ onboarding_t0:
   - drop_packets/test_drop_counters.py
 
 onboarding_t1:
+  - bgp/test_bgp_stress_link_flap.py
   - copp/test_copp.py
   - crm/test_crm.py
   - generic_config_updater/test_cacl.py