URGENT Playlist drivers tests

These tests aren't ready, but they DO fix an issue with the latest caspa which changed the way playlists' elements are created (before they were Medias, which was a mistake, now they're Pieces).

The consequence of all this, plus the way the backbone.io middlewares work, is that the pieces are now reaching us without _ids.

In this fix I use the media checksum instead of the piece id, but this might be wrong, since two pieces with same media but different filters will generate the same ID, and therefore the same XML filename, wihch I think will be very bad indeed.