Fix uv-test in `cargo-install`
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.