10005: Extend `CargoConfig.unset_test_crates` r=matklad a=regexident
This is to allow for efficiently disabling `#[cfg(test)]` on all crates (by passing `unset_test_crates: UnsetTestCrates::All`) without having to first load the crate graph, when using rust-analyzer as a library.
(FYI: The change doesn't seem to be covered by any existing tests.)
Co-authored-by: Vincent Esche <regexident@gmail.com>