compiletest: Don't allow tests with overlapping prefix names
Some tests will delete their output directory before starting. The output directory is based on the test names. If one test is the prefix of another test, then when that test starts, it could try to delete the output directory of the other test with the longer path, or otherwise clash with it while the two tests are trying to create/delete/modify the same directory.
In practice, this manifested as a random error on macOS where two tests were trying to create/delete/create `rustdoc/primitive` and `rustdoc/primitive/no_std`, which resulted in an EINVAL (InvalidInput) error.
This renames some of the offending tests, adds `compiletest-ignore-dir` to prevent compiletest from processing some files, and adds a check to prevent this from happening in the future.
Fixes#109397
