Convenience: mkdtemp a fresh bin dir under parentDir, write stubs into it,
and return its absolute path. Caller PREPENDS this dir to PATH so the fakes win
over the real binaries, and removes it when done (it lives under parentDir,
so a parentDir cleanup also clears it).
Convenience: mkdtemp a fresh bin dir under
parentDir, writestubsinto it, and return its absolute path. Caller PREPENDS this dir to PATH so the fakes win over the real binaries, and removes it when done (it lives underparentDir, so aparentDircleanup also clears it).