Add test stub that should verify that load appends playlists

tests/backends/base.py

@@ -96,6 +96,9 @@ class BaseCurrentPlaylistControllerTest(object):
         self.controller.load(Playlist())
         self.assertEqual(self.controller.version, version+1)
 
+    def test_load_appends(self):
+        raise NotImplementedError
+
     @populate_playlist
     def test_load_preserves_playing_state(self):
         tracks = self.controller.playlist.tracks