staticcheck found a couple of minor code cleanup improvements, like
unused variables or an out-of-order defer, mostly in tests.
This patch fixes those problems by making the necessary adjustments.
They're all fairly small, and should not change the logic in any
significant way.
To send mails, today some tests use msmtp and others our internal smtpc.py.
This works, but msmtp slows down the tests significantly, and smtpc.py
is also not particularly fast, and also has some limitations.
This patch introduces a new SMTP client tool written in Go, and makes
almost all the tests use it.
Some tests still remain on msmtp, mainly for client-check compatibility.
It's likely that this will be moved in later patches to a separate
special-purpose test.
With this patch, integration tests take ~20% less time than before.
