Auto merge of #74153 - ehuss:fix-bootstrap-test-librustc, r=Mark-Simulacrum

Fix x.py test for librustc crates.

#73352 introduced a bug where `x.py test src/librustc_ast` would fail to actually run the tests. The issue is that `krate` and `all_krates` were changed to return relative paths. This caused the code to do a test of "relative_path ends with absolute path" which is always false.  The solution is to swap that around.

The change to `Crate` isn't necessary, it just simplifies the code and makes it uniform with `CrateLibrustc`.