Use separate module for pytest settings (#13895)

* Use separate module for test settings

* Further refine some pre-existing comments in settings

* Add CACHES to setting snapshot exceptions to accommodate changed load order

awx/main/tests/unit/test_settings.py

@@ -8,6 +8,7 @@ LOCAL_SETTINGS = (
     'BROADCAST_WEBSOCKET_PROTOCOL',
     'BROADCAST_WEBSOCKET_SECRET',
     'DATABASES',
+    'CACHES',
     'DEBUG',
     'NAMED_URL_GRAPH',
 )