Ensure async fixture setup and teardown run in the same task

[provinzkraut]

An attempt to fix #1191.

Inspired by anyio's pytest plugin: https://github.com/agronholm/anyio/blob/b8e91a5cfe35c3b28f91ea8251674a871a7f4e26/src/anyio/_backends/_asyncio.py#L2222-L2239.

Unfortunately, this requires some hacky hacks abusing backports.asyncio.runner to ensure task context is properly propagated on Python < 3.11, in order not to break things.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 93.18182% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 89.28%. Comparing base (3ab1fea) to head (7581198).

Files with missing lines	Patch %	Lines
pytest_asyncio/plugin.py	93.18%	2 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1193      +/-   ##
==========================================
+ Coverage   88.91%   89.28%   +0.36%     
==========================================
  Files           2        2              
  Lines         406      448      +42     
  Branches       44       48       +4     
==========================================
+ Hits          361      400      +39     
- Misses         35       37       +2     
- Partials       10       11       +1     

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