Add basic gstreamer tests

tests/backends/gstreamer.py

@@ -0,0 +1,16 @@
+import unittest
+
+from mopidy.backends.gstreamer import GStreamerBackend
+
+from tests.backends import BasePlaybackControllerTest
+
+class GStreamerPlaybackControllerTest(BasePlaybackControllerTest, unittest.TestCase):
+    uri = ['file://data/song1.mp3',
+           'file://data/song2.mp3',
+           'file://data/song3.mp3',
+          ]
+
+    backend_class = GStreamerBackend
+
+if __name__ == '__main__':
+    unittest.main()