tests: Cleanup mpd.protocol.current_playlist tests
tests: Cleanup mpd.protocol.current_playlist tests

- Split into smaller test cases more or less per command
- Created a BasePopulatedTracklistTestCase with a sensible setUp
- Modified test cases to work with the common tracklist state
- Replaced all calls to tracklist.add(tracks=...) with uris=...
- Test tracklist ordering in more compact way that also gives better error
  messages

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