Detect unused files in `src/test/mir-opt` and error on them in tidy.
Closes#97564 .
Determining which files are generated by a given mir opt test is somewhat difficult. Because of this, we extract the logic for doing it out into a common crate that both compiletest and tidy can depend on. This avoids making compiletest a dependency of tidy which would negatively impact compile times for tidy.
Testing for this is that it catches 5 files that violated this lint (and removes them).
e2a6d283be