In the previous iteration, if we were ever down/frozen/disabled long enough
to miss sending two mails in a row, we would see unintended "catch-up"
behavior where each subsequent run of britney would send a mail until the
right total number of mails had been sent. Don't do this; instead, catch us
up in one go to the most recent mail that should have been sent, avoiding
bunching of notifications.
This changes one of the tests also to match.
I want to fix two bugs in interactions between other parts of britney
and the email policy. It's not currently easy to do so because we just
run the policy itself manually by creating some fake excuses.
Steal part of the machinery from the autopkgtest tests, and run a few
tests through britney completely. Use a fake SMTP server to record which
emails we sent.
(The port is hardcoded - that might not be so smart.)