Fix new warnings assertions to use pytest.warns()

[mgorny]

Change Summary

Fix new tests to use the correct pytest.warns() function to assert for warnings, rather than pytest.raises(). This fixes testing without -Werror.

Related issue number

n/a

Checklist

• The pull request title is a good summary of the changes - it will be used in the changelog
• Unit tests for the changes exist
• Tests pass on CI
• Documentation reflects the changes where applicable
• My PR is ready to review, please add a comment including the phrase "please review" to assign reviewers

Selected Reviewer: @sydney-runkle

[codspeed-hq]

CodSpeed Performance Report

Merging #10241 will not alter performance

_{Comparing mgorny:test-warn-assert (dd7ed4b) with main (714c0df)}

Summary

✅ 17 untouched benchmarks

[github-actions]

Coverage report

This PR does not seem to contain any modification to coverable code.

[mgorny]

please review

[Viicos]

Thanks for the contribution

[mgorny]

Thanks.