[WIP] Update tests for transactional email routing in mailforwarder

[Copilot]

Thanks for asking me to work on this. I will get started on it and keep this PR's description up to date as I form a plan and make progress.

You're operating on my authserver repo. That's an OpenID Connect server based on Python/Django which also supports my email setup through two daemons
mailforwarder and dkimsigner). I recently added support for transactional email routing, where transactional emails take a different path through
authserver/maildaemons/forwarder/server.py when they have the can_use_transactional_email flag set on the Domain model (see authserver/mailauth/
models.py ). However, I didn't update the tests in authserver/mailauth/tests/test_mailforwarder_srs.py and now they're failing. They rely on old code that
I removed from mailforwarder and also test old logic that doesn't exist anymore. Please update them. Ask any questions necessary.

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