backend: Add test for get_images fallback

tests/backend/test_backend.py

@@ -0,0 +1,19 @@
+from __future__ import absolute_import, unicode_literals
+
+import unittest
+
+from mopidy import models
+
+from tests import dummy_backend
+
+
+class LibraryTest(unittest.TestCase):
+    def test_default_get_images_impl_falls_back_to_album_image(self):
+        album = models.Album(images=['imageuri'])
+        track = models.Track(uri='trackuri', album=album)
+
+        library = dummy_backend.DummyLibraryProvider(backend=None)
+        library.dummy_library.append(track)
+
+        expected = {'trackuri': [models.Image(uri='imageuri')]}
+        self.assertEqual(library.get_images(['trackuri']), expected)