- user: add `base_url` parameter for user auth client calls
* Adds a `base_url` parameter to the UserFixtureBase constructor. This
allows to make sure that auth calls like login and logout are
explicitly associated with the correct domain when needed. This is
avoid cookie domain inconsistencies intorduced by Flask v3 and
Werkzeug's v2.3 new default behavior for cookie management in the test
client.