mpd: Test that "list album foo" responds correctly

Previously, test_list_album_with_artist_name would only test that the
command didn't fail. Now it also checks that the response is correct.
That is, that the response contains albums.

This makes the test detect the error reported in #817.

tests/mpd/protocol/test_music_db.py

@@ -880,7 +880,11 @@ class MusicDatabaseListTest(protocol.BaseTestCase):
         self.assertInResponse('OK')
 
     def test_list_album_with_artist_name(self):
+        self.backend.library.dummy_find_exact_result = SearchResult(
+            tracks=[Track(album=Album(name='foo'))])
+
         self.sendRequest('list "album" "anartist"')
+        self.assertInResponse('Album: foo')
         self.assertInResponse('OK')
 
     def test_list_album_with_artist_name_without_filter_value(self):