Stabilize test_snmp_fdb_send_tagged

[lolyu]

Description of PR

Summary:
Fixes # (issue)

Type of change

• Bug fix
• Testbed and Framework(new/improvement)
• Test case(new/improvement)

Back port request

• 202012
• 202205
• 202305
• 202311
• 202405
• 202411

Approach

What is the motivation for this PR?

test_snmp_fdb_send_tagged is flaky on dualtor testbed.

Signed-off-by: Longxiang Lyu [email protected]

How did you do it?

Two improvements:

1. flush fdb table on the correct DUT.
2. check the fdb table first to ensure the fdb table is correctly populated before checking snmp, also increase the timeout.

How did you verify/test it?

collected 1 item

snmp/test_snmp_fdb.py::test_snmp_fdb_send_tagged PASSED                                                                                                                                                                                                                [100%]

================================================================================================================== 1 passed, 1 warning in 377.48s (0:06:17) ==================================================================================================================

Any platform specific information?

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

Documentation

[mssonicbld]

/azp run

[azure-pipelines]

Azure Pipelines successfully started running 1 pipeline(s).

[StormLiangMS]

LGTM

[mssonicbld]

/azp run

[azure-pipelines]

Azure Pipelines successfully started running 1 pipeline(s).

[mssonicbld]

Cherry-pick PR to 202405: #16502