Cleanup model ref resetting.

moto/core/models.py

@@ -466,10 +466,14 @@ class BaseModel(object):
 
 class BaseBackend(object):
 
-    def reset(self):
+    def _reset_model_refs(self):
+        # Remove all references to the models stored
         for service, models in model_data.items():
             for model_name, model in models.items():
                 model.instances = []
+
+    def reset(self):
+        self._reset_model_refs()
         self.__dict__ = {}
         self.__init__()