Fix test_slash_l* to support non-en_US locales

[mgorny]

Description

Update the tests to expect the current locale rather than en_US. While technically this isn't 100% guaranteed to be the same locale as the database is running on, I think we can reasonably assume that for the purpose of testing they are the same. Non-UTF-8 locales still are not supported but the other tests seem to be unable to handle them well anyway.

Fixes #140

Checklist

• [x] I've added this contribution to the changelog.rst.
• [ ] I installed pre-commit hooks (pip install pre-commit && pre-commit install), and ran black on my code.
• [x] Please squash merge this pull request (uncheck if you'd like us to merge as multiple commits)

[mgorny]

Damn, I knew this is going to happen — the Docker container is actually using a different locale than GHA environment. I'm going to try if I can set them to match.

[j-bennet]

Thank you @mgorny.

[mgorny]

Thanks!