Test improvements for DSF.

- Fixed unparseable.dsf
- Added DSF feature detection to test_mediafile.py

test/test_mediafile.py

@@ -886,8 +886,16 @@ class AIFFTest(ReadWriteTestBase, unittest.TestCase):
         'channels': 1,
     }
 
+try:
+    import mutagen.dsf
+except:
+    HAVE_DSF = False
+else:
+    HAVE_DSF = True
 
-class DSFTest(ReadWriteTestBase, unittest.TestCase):
+@unittest.skipIf(not HAVE_DSF, "mutagen < 1.37")
+class DSFTest(ReadWriteTestBase,
+              ExtendedImageStructureTestMixin, unittest.TestCase):
     extension = 'dsf'
     audio_properties = {
         'length': 0.01,