tests: Simplify OIDCUser tests
Remove the AppUser test model inheriting from OIDCUser as this is not
how the model should be used (composition should be preferred).
Update tests accordingly.
Migrated from D5331 (view on Phabricator)
Remove the AppUser test model inheriting from OIDCUser as this is not
how the model should be used (composition should be preferred).
Update tests accordingly.
Migrated from D5331 (view on Phabricator)