compiletest can fail spuriously when the same file is an aux file and a test file by itself

This is a problem because when there are multiple test threads and multiple rustc processes, it's possible for two rustc processes to fs race -- one thread might be trying to build the aux'd main test as a main test, the other might be trying to build the aux'd main test as an aux... And other interesting behaviors.

We'd probably need to do something like:

• (1) Have one of the main test under question not aux-build another main test.
  • https://github.com/rust-lang/rust/pull/144246
• (2) See if we can "quarantine" aux build directories, as multiple test threads could have multiple rustc processes trying to... produce artifacts in the same aux build directory.
• (3) As part of the longer-term changes, see if we can perform some kind of build graph validation to reject cases like this.