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-16-spf/expected_dsn

@@ -54,8 +54,6 @@ Received: from localhost
 	tls *
 	(over *
 	; *
-From: userA@srv-A
-Date: *
 Subject: Prueba desde el test
 
 Crece desde el test el futuro

test/t-16-spf/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-16-spf/run.sh

@@ -54,7 +54,7 @@ function launch_minidns() {
 
 # T0: Successful.
 launch_minidns zones.t0
-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
 
@@ -62,7 +62,7 @@ mail_diff content .mail/userb@srv-b
 # Check that userA got a DSN about it.
 rm .mail/*
 launch_minidns zones.t1
-run_msmtp userB@srv-B < content
+smtpc userB@srv-B < content
 wait_for_file .mail/usera@srv-a
 mail_diff expected_dsn .mail/usera@srv-a
 

test/t-16-spf/smtpc.conf

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