Speed-up test collection for larger codebases

[fahhem]

Changes a couple places to check asyncio_mode (cheap) before _is_asyncio_fixture_function

To fix #720

[fahhem]

Might also close #997

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 0% with 1 line in your changes missing coverage. Please review.

Project coverage is 91.42%. Comparing base (38ac413) to head (d5658f5).
Report is 18 commits behind head on main.

Files with missing lines	Patch %	Lines
pytest_asyncio/plugin.py	0.00%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@ Coverage Diff @@ ## main #1020 +/- ## ======================================= Coverage 91.42% 91.42% ======================================= Files 2 2 Lines 548 548 Branches 72 72 ======================================= Hits 501 501 Misses 28 28 Partials 19 19 

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[asvetlov]

LGTM in general.
Could you please tune the title to stress the PR main intention: it speed-ups test collection for very large code base.
The title will be a part of changelog at the end.

[fahhem]

done, thanks!

[asvetlov]

Thanks!

[seifertm]

We currently don't have a mechanism that automatically updates the changelog when a PR is merged.
Therefore I added a corresponding changelog entry for this patch in a4e82ab