[action] [PR:21320] fix: disable route check for po cleanup test cases

[mssonicbld]

Description of PR

We are having a module level pytest.mark.disable_route_check marker in pc/test_po_cleanup.py test to temporarily disable the routeCheck monitor for the entire module. However, we are doing config_reload() in different test cases within this test module, and the routeCheck monitor will automatically startup after each config_reload().

Therefore, we want to keep the module level routeCheck disable marker (so the routeCheck monitor will always be started at the very end of the test module) and introduce an extra function-level routeCheck disable fixture to make sure the routeCheck monitor is always disabled before each test case.

Summary:
Fixes # (issue) Microsoft ADO 35884974

Type of change

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

Back port request

• 202205
• 202305
• 202311
• 202405
• 202411
• 202505

Approach

What is the motivation for this PR?

Disable the routeCheck monitor before each test case in pc/test_po_cleanup.py to avoid getting any noisy error syslog.

How did you do it?

Introduced a function-level fixture to disable routeCheck monitor.

How did you verify/test it?

I ran the updated code and can confirm it's working well: https://elastictest.org/scheduler/testplan/6916acddbf375d1f9e5d1333

Any platform specific information?

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

Documentation

[mssonicbld]

Original PR: #21320

[mssonicbld]

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