Replace assertEqual(None, *) with assertIsNone in tests

Replace assertEqual(None, *) with assertIsNone in tests to have
more clear messages in case of failure.

Change-Id: I2cc0d913d83ea56c323475daddb66a0047564b42

os_collect_config/tests/test_cache.py

@@ -99,4 +99,4 @@ class TestCache(testtools.TestCase):
         self.assertFalse(changed)
 
     def test_commit_no_cache(self):
-        self.assertEqual(None, cache.commit('neversaved'))
+        self.assertIsNone(cache.commit('neversaved'))