Improved how references test handles paths

[RunDevelopment]

This is a PR for the improvements I made in #4216.

Changes:

1. All paths are now relative to the repo root instead of cwd. This means that the test will work no matter the cwd. I did this to make debuggers work with references.rs without any config. Debuggers in VSCode can be configured to use a different cwd, but that would require creating a .vscode/launch.json. Now the default debugger just works.
2. The compiler output is longer in /target but /target/tests/reference. The old target messed up incremental compilation for references.rs itself, which meant that running references.rs always meant compiler 50 or so deps before every run. On my old laptop, this means that running references.rs was 2 minutes of compiling and 30 sec of running tests, every time.

[daxpedda]

Nice, thank you!