Fix uv-test in `cargo-install` (#17954)
The uv crate has uv-test as dev-dependency. We want that by default, uv
runs tests with `uv-test/git`, so it's a default feature in uv. The
problem is that uv-test is only a dev dependency, which gets stripped by
`cargo install`, and `cargo install` fails with a missing crate error.
As workaround, we make `uv-test` a regular optional dependency. It's
never used as a regular dependency, but it allows using `uv-test?/git`
as default feature.
Closes #17908
Fixes #17955
