Report dead snapshots for tests that still exist

[mjambon]

Scenario:

1. I decide that a test should capture stdout. I commit the snapshots on my git branch.
2. I change my mind and make the test capture nothing or maybe capture stderr instead.
3. The test program should report the snapshot files that are no longer in use but it doesn't.

Expectation:

• ./test status should report that the stdout file containing the snapshot is no longer in use and can be removed.
• --autoclean should remove the dead snapshot files (even if their folder remains because say we now capture stderr).

Importance:

• This is not critical. Accidentally merging my dead snapshots into my project's main branch is unlikely to happen because we review the list of files before merging.
• It's useful for cleaning up easily.
• It would match user expectations.