Fix build break with Fixtures 1.3

Our explicit call to cleanUp messes things up in latest
fixture, so we need to call _clear_cleanups to stop
the test from breaking

Change-Id: I8ce2309a94736b47fb347f37ab4027857e19c8a8

tests/unit/test_moxstubout.py

@@ -30,3 +30,4 @@ class TestMoxStubout(base.BaseTestCase):
         f.cleanUp()
         after2 = TestMoxStubout._stubable
         self.assertEqual(before, after2)
+        f._clear_cleanups()