Added python fixture for resetting config changes to fix nightly backend snmp test failures

[developfast]

Description of PR

Summary:
In the nightly tests, there were snmp failures on the backend T0 and T1, the cause of which was related to config issues with the DUT, specifically the queue counterpoll being disabled by another test beforehand. To solve this issue, before any of the snmp tests are executed, the queue counterpoll is enabled in a python fixture in the associated conftest.py file.
Fixes # (issue)

Type of change

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

Back port request

• 201911
• 202012
• 202205

Approach

What is the motivation for this PR?

Fix snmp test failures occuring in nightly tests on backend T0 and T1

How did you do it?

Adding a python fixture that enables the queue counterpoll before the snmp tests are run

How did you verify/test it?

Ran the snmp test suite on a DUT in the lab, with the config disabled initially to see that the tests pass

Any platform specific information?

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

Documentation

[azure-pipelines]

The pre-commit check detected issues in the files touched by this pull request.
The detected issues may be old or new. For new issues, please try to fix them.

For old issues, it is not mandatory to fix them because they were not caused by this change. It is unfair to blame
author of this pull request. But if you can take extra effort to fix the old issues as well, that would be great!

Detailed pre-commit check results:
trim trailing whitespace.................................................Passed
fix end of files.........................................................Passed
check yaml...........................................(no files to check)Skipped
check for added large files..............................................Passed
check python ast.........................................................Passed
flake8...................................................................Failed
- hook id: flake8
- exit code: 1

tests/snmp/conftest.py:4:1: E302 expected 2 blank lines, found 1
tests/snmp/conftest.py:9:1: E302 expected 2 blank lines, found 1
tests/snmp/conftest.py:14:1: E302 expected 2 blank lines, found 1

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