Centralize test setup logic

[seifertm]

This patch moves logic for test setup from a pytest hook to the respective pytest-asyncio test item classes.

Compared to performing the same logic in a pytest hook, this approach centralizes the logic for test execution and makes it easier to follow the program flow. The fact that the setup code executes inside the concrete subclass of PytestAsyncioFunction rather than a generic pytest.Item leads to fewer safety checks as an added benefit.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 88.88889% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 90.81%. Comparing base (c68337e) to head (c9f2eb9).

Files with missing lines	Patch %	Lines
pytest_asyncio/plugin.py	88.88%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1214      +/-   ##
==========================================
- Coverage   90.86%   90.81%   -0.05%     
==========================================
  Files           2        2              
  Lines         405      403       -2     
  Branches       44       43       -1     
==========================================
- Hits          368      366       -2     
  Misses         29       29              
  Partials        8        8              

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