[action] [PR:18081] Fix for lag port selection in test_bgp_update_timer.py (#353)

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### Description of PR
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The test_bgp_update_timer.py selects the ports via random port selection in setup. The fixture 'common_setup_teardown' fails the test if the ports belong to different asics for mutli-asic chassis.
Hence updated the function '_setup_interfaces_t1_or_t2' to apply random selection on the set of interfaces belonging to same asic.
Summary:
Fixes # (issue)

### Type of change

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- [x] Bug fix
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- [ ] Bug fix
- [ ] Testbed and Framework(new/improvement)
- [ ] New Test case
 - [ ] Skipped for non-supported platforms
- [x] Test case improvement

### Back port request
- [ ] 202012
- [ ] 202205
- [ ] 202305
- [ ] 202311
- [ ] 202405
- [x] 202411

### Approach
#### What is the motivation for this PR?
Test failure due to selected ports belonging to different asics
#### How did you do it?

#### How did you verify/test it?

#### Any platform specific information?

#### Supported testbed topology if it's a new test case?

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Author: mssonicbld

tests/bgp/conftest.py

@@ -346,7 +346,6 @@ def _setup_interfaces_t1_or_t2(mg_facts, peer_count):
                         if not ipv4_interfaces:
                             ipv4_interfaces.append(intf["attachto"])
                             asic_idx = intf_asic_idx
-                            used_subnets.add(ipaddress.ip_network(intf["subnet"]))
                         else:
                             if intf_asic_idx != asic_idx:
                                 continue
@@ -359,21 +358,21 @@ def _setup_interfaces_t1_or_t2(mg_facts, peer_count):
                 for pt in mg_facts["minigraph_portchannel_interfaces"]:
                     if _is_ipv4_address(pt["addr"]):
                         pt_members = mg_facts["minigraph_portchannels"][pt["attachto"]]["members"]
+                        pc_asic_idx = duthost.get_asic_index_for_portchannel(pt["attachto"])
                         # Only use LAG with 1 member for bgpmon session between PTF,
                         # It's because exabgp on PTF is bind to single interface
                         if len(pt_members) == 1:
                             # If first time, we record the asic index
-                            if not ipv4_lag_interfaces:
+                            if not ipv4_interfaces and not ipv4_lag_interfaces:
+                                asic_idx = pc_asic_idx
                                 ipv4_lag_interfaces.append(pt["attachto"])
-                                asic_idx = duthost.get_asic_index_for_portchannel(pt["attachto"])
-                            # Not first time, only append the portchannel that belongs to the same asic in current list
+                            # Not first time, only append the port-channel that belongs to the same asic in current list
                             else:
-                                asic = duthost.get_asic_index_for_portchannel(pt["attachto"])
-                                if asic != asic_idx:
+                                if pc_asic_idx != asic_idx:
                                     continue
                                 else:
                                     ipv4_lag_interfaces.append(pt["attachto"])
-                        used_subnets.add(ipaddress.ip_network(pt["subnet"]))
+                            used_subnets.add(ipaddress.ip_network(pt["subnet"]))
 
             vlan_sub_interfaces = []
             if is_backend_topo: