[GCU] Update the testcase to avoid duplicate ip range (sonic-net#16816)

What is the motivation for this PR?
Patch Applier: * [{"op": "remove", "path": "/BGP_PEER_RANGE/BGPSLBPassiveV6"}]\n
Patch Applier: * [{"op": "add", "path": "/BGP_PEER_RANGE/BGPVac", "value": {"ip_range": ["192.168.0.0/21"], "name": "BGPVac", "src_address": "10.1.0.32"}}]\n
Patch Applier: * [{"op": "remove", "path": "/BGP_PEER_RANGE/BGPSLBPassive"}]\n
Patch Applier: * [{"op": "add", "path": "/BGP_PEER_RANGE/BGPSLBPassive", "value": {"ip_range": ["10.255.0.0/25"], "name": "BGPSLBPassive", "src_address": "10.1.0.32"}}]

The problem comes from that the running config already has 192.168.0.0 in BGPSLBPassive when add ip_range 192.168.0.0 to BGPVac. Then it cause the ip_range duplicate issue. Though the BGPSLBPassive's ip range change also in the patch, it comes after BGPVac which cause issue during the rollback.
The fix is to clean up the config before rollback.

How did you do it?
Remove BGP_PEER_RANGE config before rollback.

How did you verify/test it?
E2E

Author: wen587

tests/generic_config_updater/test_bgp_speaker.py

@@ -261,3 +261,4 @@ def test_bgp_speaker_tc1_test_config(rand_selected_dut, lo_intf_ips, vlan_intf_i
     bgp_speaker_tc1_add_dummy_ip_range(rand_selected_dut)
     bgp_speaker_tc1_rm_dummy_ip_range(rand_selected_dut)
     bgp_speaker_tc1_replace_src_address(rand_selected_dut)
+    bgp_speaker_config_cleanup(rand_selected_dut)