Rendering any admin page stamps the sidebar's managed-models set onto

the registered AdminManageNavGroupAction singleton (inside

AdminSidebarManageActionGroupRenderer._get_sidebar_state()),

and that instance attribute survived across tests. Once any earlier test

rendered admin chrome, the "unmanaged" cache tests saw User as managed,

the post_save/post_delete handlers invalidated the cache, and the tests

failed.
The test class now clears the stamped state from the registered action

in setUp(), and _patch_managed_models() patches the registered

instance instead of the class, which the instance attribute would

otherwise shadow.

Ran unit tests in random order.