Fixing the failing test.

TNL-900

openassessment/xblock/test/test_staff_info.py

@@ -523,7 +523,7 @@ class TestCourseStaff(XBlockHandlerTestCase):
         xblock.xmodule_runtime = self._create_mock_runtime(
             xblock.scope_ids.usage_id, False, False, "Bob"
         )
-        resp = self.request(xblock, 'cancel_submission', json.dumps({}))
+        resp = self.request(xblock, 'cancel_submission', json.dumps({'submission_uuid': "Bob"}))
         self.assertIn("you do not have permission", resp.decode('utf-8').lower())
 
         # If we ARE course staff, then we should see the cancel submission option