Tests: Change all instances of mail.outbox to mailoutbox fixture
Self-explanatory from title.
There is something weird going on, though. Apparently, there is a bug with the mailoutbox
fixture, where it needs to be the last function parameter to work properly:
https://stackoverflow.com/questions/66846621/mailoutbox-works-only-if-last-fixture
It's fine now but it tripped me up.