Auto merge of #124296 - cuviper:dist-cargo-tests, r=onur-ozkan

bootstrap: keep all cargo test files in dist rustc-src

Cargo tests use some files that we would otherwise exclude, especially
the `cargo init` tests that are meant to deal with pre-existing `.git`
and `.hg` repos and their ignore files. Keeping these in our dist
tarball doesn't take much space, and allows distro builds to run these
tests successfully.

Author: bors