Automate WhisperBack tests

We may not want to automate this test because it would spam tails-bugs@, and parts of it requires manual verification from someone there. Still, it would be relevant to test that WhisperBack doesn’t itself detects that it fails to send the email, because that indicates a definite failure (i.e. that our hidden service is down, but perhaps that’s better tested by some external monitoring tool?).

Alternatively, if we want such a test, we could tag it @release-testing, and somehow make it so that scenarios tagged @release-testing are only run when the test suite is invoked with a --release-testing option (or @ … — —tag release-testing), intended to only be used when testing a release (this one could also ensure that --iso != --old-iso, etc.).

Parent Task: #10250

Original created by @anonym on 10270 (Redmine)