test: Unify (most) SMTP client calls

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.

test/t-14-tls_tracking/msmtprc

@@ -1,14 +0,0 @@
-account default
-
-host srv-A
-port 1587
-
-tls on
-tls_trust_file A/certs/srv-A/fullchain.pem
-
-from userA@srv-A
-
-auth on
-user userA@srv-A
-password userA
-

test/t-14-tls_tracking/run.sh

@@ -45,7 +45,7 @@ wait_until_ready 1025
 wait_until_ready 2025
 wait_until_ready 9053
 
-run_msmtp userB@srv-B < content
+smtpc userB@srv-B < content
 
 wait_for_file .mail/userb@srv-b
 mail_diff content .mail/userb@srv-b

test/t-14-tls_tracking/smtpc.conf

@@ -0,0 +1,4 @@
+addr localhost:1465
+server_cert A/certs/srv-A/fullchain.pem
+user userA@srv-A
+password userA