Updated the _getobj method to fix TypeError in pytest 7.0.0

[fumedev]

COMPLETED TASK

• Modified the _get_obj method in the pytest.Function class within the src/_pytest/unittest.py file to provide a default value for the test_method argument during instance creation. This change prevents the TypeError raised due to a missing required positional argument introduced in pytest 7.0.0.
• Updated the lines within the _get_obj method to include a default value for test_method, ensuring the passed argument matches the original intent within the class.
• Tested the changes internally to verify the fix and ensure no regressions were introduced.
• Created a Pull Request with detailed information about the problem, analysis, and how the proposed change resolves the TypeError issue in pytest 7.0.0.
• Pending code review for further feedback and possible merging based on reviewer's comments.