d14816cbaf
- Tests use a session-scoped mock stage directory so as not to interfere
with the real install.
- Every test is forced to clean up after itself with an additional check.
We now automatically assert that no new files have been added to
`spack.stage_path` during each test.
- This means that tests that fail installs now need to clean up their
stages, but in all other cases the check is useful.