Since the public interface doesn't expose a save() method, those changes
would never be changed

So far, we only handle the case where a Media has multiple files

This is now only part of the test implementation, and allows us to
remove quite a bit of overhead

Instead of re-computing it dynamically while it's never going to change

As a result, the Media.Duplicate test is obsolete, since a filename
isn't unique in itself. Only the couple (mrl, folder_id) is UNIQUE.
However, UNIQUE doesn't apply when the folder_id foreign key is NULL
Uniqueness has been manually tested for a real folder/mrl couple

Instead of having a per-field SQL request.

Only use a Media instance & expose media deletion methods internally
only

Media now always have a single artist
Artist are being added media
The album artist meta is used to generate the list of artists
The artist meta isn't used for artists list

Use a default FS mock for all tests
Do not duplicate setup code