SEBA-466 Fix or note potential concurrency issues

Change-Id: Iabfa6c7021cacc565c39e09fcd884fa048ca5004
diff --git a/xos/synchronizer/model_policies/test_model_policy_voltserviceinstance.py b/xos/synchronizer/model_policies/test_model_policy_voltserviceinstance.py
index 3ded41d..1cfdf05 100644
--- a/xos/synchronizer/model_policies/test_model_policy_voltserviceinstance.py
+++ b/xos/synchronizer/model_policies/test_model_policy_voltserviceinstance.py
@@ -129,7 +129,7 @@
             self.policy.associate_onu_device(self.si)
 
             self.assertEqual(self.si.onu_device_id, mock_onu.id)
-            self.si.save.assert_called()
+            self.si.save_changed_fields.assert_called()
 
     def test_handle_delete(self):
         self.policy.handle_delete(self.si)