[internal] Register `platform_specific_behavior` marker with Pytest (#12679)
[internal] Register `platform_specific_behavior` marker with Pytest (#12679)

Follow up on https://github.com/pantsbuild/pants/pull/12665#issuecomment-907369756. While custom markers work even if they're not registered, it results in a warning (as found by @stuhood):

>   /private/var/folders/bg/_r10hqp14kjcpv68yzdk5svc0000gn/T/process-executionaB8Qfx/src/python/pants/backend/python/typecheck/mypy/rules_integration_test.py:107: PytestUnknownMarkWarning: Unknown pytest.mark.platform_specific_behavior - is this a typo?  You can register custom marks to avoid this warning - for details, see https://docs.pytest.org/en/stable/mark.html
    @pytest.mark.platform_specific_behavior

Per https://docs.pytest.org/en/stable/mark.html#registering-marks, we should register this.

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