Report snapshot files that moved outside Testo's private folders

[mjambon]

This is a similar issue as #129 but in this situation, we want to report snapshot files that exist in the standard location but are unused because a custom location was newly specified with Testo.stdout ~expected_stdout_path () or similar.

It would be great if we could provide a way to move these files for the user. Like we have run --autoclean for deleting unused files, we could add an --automove option. Both autoclean and automove options could be extended in the future to be a little smarter. For example, detecting when a test was renamed and migrating its snapshots would be useful, a bit like how Git detects renamed files.

[mjambon]

It turns out to be complicated to provide a clean future-proof feature that just does the right thing with old snapshot files. This is why I ended up just adding an --autoclean option to the run subcommand initially. Challenges include:

• after a test rename or snapshot file rename, moving the snapshots before running the tests -> requires dedicated subcommand
• identifying renamed files:
  • approximate contents matching requires more elaborate algorithms that exact matching
  • ... and doing so efficiently as the number of existing tests increases requires keeping hashes and indexes in files
  • ... but adding many files makes everything less performant so it's best to keep things in somewhat large index files rather than one file per test
  • ... which requires a decent format for storing things. We could use JSON but it adds a dependency on a library which is not ideal (yojson doesn't depend on much but many people prefer fewer dependencies)

[mjambon]

If you need to identify duplicate snapshots, I suggest using the fclones command:

fclones group .

will report lists of identical files.