Fix base_manager unit test

As more methods are added to the PolymorphicModel, the attr dict changes
ordering in the meta class. By making the ordering of managers
consistent, this problem no longer occurs.

polymorphic/base.py

@@ -140,6 +140,11 @@ class PolymorphicModelBase(ModelBase):
                     self.validate_model_manager(manager, self.__name__, key)
                 add_managers.append((base.__name__, key, manager))
                 add_managers_keys.add(key)
+
+        # The ordering in the base.__dict__ may randomly change depending on which method is added.
+        # Make sure base_objects is on top, and 'objects' and '_default_manager' follow afterwards.
+        # This makes sure that the _base_manager is also assigned properly.
+        add_managers = sorted(add_managers, key=lambda item: item[2].creation_counter, reverse=True)
         return add_managers
 
     @classmethod