Test that static manifiest file does not get deleted

A bug in the process which cleans up unhashed and intermediate files is
also deleting the manifest file, which is obviously not ideal.

tests/test_storage.py

@@ -72,3 +72,7 @@ def test_unversioned_files_are_deleted(self):
                 f for f in os.listdir(settings.STATIC_ROOT)
                 if name_pattern.match(f)]
         self.assertEqual([versioned_name], remaining_files)
+
+    def test_manifest_file_is_left_in_place(self):
+        manifest_file = os.path.join(settings.STATIC_ROOT, 'staticfiles.json')
+        self.assertTrue(os.path.exists(manifest_file))