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.
This patch adds dovecot support to the chasquid daemon, using the
internal dovecot library added in previous patches.
Dovecot support is still considered EXPERIMENTAL and may be reverted, or
changed in backwards-incompatible ways.
The patch also adds the corresponding integration test, which brings up
a dovecot server with a custom configuration, and tests chasquid's
authentication against it. If dovecot is not installed, the test is
skipped.
