Switch to use uv.lock instead of test-requirements.txt

[CoolCat467]

In this pull request, we switch the CI system to use uv.lock instead of test-requirements.txt.

[A5rocks]

I don't like this change (because I install the dependencies using pip install -r test-requirements.txt!), so I'm a -1 here.

My opinion might shift if there's ever a standardized lockfile format though.

[jakkdl]

I don't like this change (because I install the dependencies using pip install -r test-requirements.txt!), so I'm a -1 here.

My opinion might shift if there's ever a standardized lockfile format though.

presumably one could use uv sync tests for the same effect? I think that might be faster and stuff. But afaict the current setup works fairly well so I don't have strong opinions either way - maybe time to revisit #2790 and see which, if any, of the pain points brought up are still a thing.