* add functionality for filtering mail log by ID
This was not planned, but as @polarthene mentioned in
https://github.com/docker-mailserver/docker-mailserver/pull/3033#issuecomment-1407169569
, filtering the mail log by email ID would be (the only) correct
approach for the Rspamd test (to eliminate race conditions).
I asserted the currect state, and came to the conclusion that this might
(or actually is) something we want in more than one place. So I went
ahead and implemented a solution.
The solution for acquiring the ID is a bit slower because it ensures the
mail queue is empty _before_ and _after_ the mail is sent. This is the
tradeoff one has to make if they want to send multiple emails in one
test file and get their IDs.
I hope you like this approach. I will provide another PR that adjusts
our current tests to use these new functions.
* added note about our helper functions in the docs
I think our work for our custom test framework should be noted in the
docs for newcomers to better understand what they should do.
* adjust Rspamd test to use new helpers for sending
* improve filter helpers further
* add sanity check when acquiring mail ID
* re-add `refute_output` to test which should now work well
