uv/tests: add new 'ecosystem' integration tests (#5970)
At a high level, this PR adds a smattering of new tests that
effectively snapshot the output of `uv lock` for a selection of
"ecosystem" projects. That is, real Python projects for which we expect
`uv` to work well with.
The main idea with these tests is to get a better idea of how changes
in `uv` impact the lock files of real world projects. For example,
we're hoping that these tests will help give us data for how #5733
differs from #5887.
This has already revealed some bugs. Namely, re-running `uv lock` for a
second time will produce a different lock file for some projects. So to
prioritize getting the tests added, for those projects, we don't do the
deterministic checking.
