Fixed incorrect default argument so that tests run; however the new value is…

Fixed incorrect default argument so that tests run; however the new value is incorrect and needs to be fixed.

common/lib/xmodule/xmodule/tests/__init__.py

@@ -31,7 +31,7 @@ i4xs = ModuleSystem(
     user=Mock(),
     filestore=fs.osfs.OSFS(os.path.dirname(os.path.realpath(__file__))),
     debug=True,
-    xqueue_callback_url='/',
+    xqueue=None, # TODO FIXME
     is_staff=False
 )