This commit introduces a new task variable RUST_MANIFEST_DIRNAME which
points at the parent directory of the manifest for the current package.
Previously we were running `cargo test` inside of parent dir of a
currently focused source file, which happened to work with non-debug
stuff (as `cargo test` itself fixed cwd for us), but that no longer
works with debug scenarios - they are compiled separately and so we no
longer have cargo doing the heavy lifting for us

Co-authored-by: Remco Smits <djsmits12@gmail.com>
Co-authored-by: Anthony Eid <hello@anthonyeid.me>

Closes #33751

Release Notes:

- debugger: Fixed wrong cwd in automatically-generated Rust test debug
sessions

Co-authored-by: Remco Smits <djsmits12@gmail.com>
Co-authored-by: Anthony Eid <hello@anthonyeid.me>