fix compatibility for pytest 8

[haifeng-jin]

What does this PR do?

The test test_seed_stays_same_with_multiple_seed_everything_calls is only compatible with pytest==7, but not pytest==8 due to its passing None to pytest.warns(), which is deprecated.

This PR fixes the test by making it compatible with both pytest 7 & 8.

more context:
We (Google) uses a monorepo that enforces the pytest version to be 8 for now.
We want to be able to run the unit tests instead of just importing the code.
So, we would like to make this compatibility fix.

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• Did you update the CHANGELOG? (not for typos, docs, test updates, or minor internal changes/refactors)

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📚 Documentation preview 📚: https://pytorch-lightning--20575.org.readthedocs.build/en/20575/

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 49%. Comparing base (ea59e40) to head (d55923d).
Report is 5 commits behind head on master.

❗ There is a different number of reports uploaded between BASE (ea59e40) and HEAD (d55923d). Click for more details.

HEAD has 2265 uploads less than BASE

Flag	BASE (ea59e40)	HEAD (d55923d)
cpu	525	24
lightning_fabric	67	7
pytest	266	26
python3.9	132	6
lightning	394	19
python3.10	66	3
python3.11	132	6
python3.12.7	195	9
pytorch2.1	99	0
pytest-full	261	0
pytorch2.2.2	33	0
pytorch_lightning	66	0
pytorch2.3	33	0
pytorch2.4.1	31	0
pytorch2.5.1	65	0

Additional details and impacted files

@@            Coverage Diff            @@
##           master   #20575     +/-   ##
=========================================
- Coverage      88%      49%    -39%     
=========================================
  Files         267      264      -3     
  Lines       23380    23324     -56     
=========================================
- Hits        20481    11382   -9099     
- Misses       2899    11942   +9043     

[haifeng-jin]

Hi @Borda , is there a next-step for me to get this PR merged? Thanks!