Use setUpTestData() to speed up tests

Pylint's `bad-classmethod-argument` is disabled for each definition
to avoid rewriting the method bodies just to rename `self` → `cls`.
This can be done gradually, as the setUpTestData methods are modified
along the way.

pytest.ini

@@ -6,6 +6,7 @@ markers =
     integration: marks tests as requiring external resources (deselect with '-m "not integration"')
 
 env =
+    LANGUAGE_CODE = en-US
     SECRET_KEY = beepbeep
     DEBUG = false
     USE_HTTPS = true