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-15-driusan_dkim/config/chasquid.conf

@@ -1,5 +1,6 @@
 smtp_address: ":1025"
 submission_address: ":1587"
+submission_over_tls_address: ":1465"
 monitoring_address: ":1099"
 
 mail_delivery_agent_bin: "test-mda"

test/t-15-driusan_dkim/msmtprc

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

test/t-15-driusan_dkim/run.sh

@@ -23,11 +23,14 @@ add_user someone@testserver secretpassword
 
 mkdir -p .logs
 chasquid -v=2 --logfile=.logs/chasquid.log --config_dir=config &
-wait_until_ready 1025
+wait_until_ready 1465
 
 # Authenticated: user@testserver -> someone@testserver
 # Should be signed.
-run_msmtp someone@testserver < content
+smtpc --addr=localhost:1465 \
+	--server_cert=config/certs/testserver/fullchain.pem \
+	--user=user@testserver --password=secretpassword \
+	someone@testserver < content
 wait_for_file .mail/someone@testserver
 mail_diff content .mail/someone@testserver
 grep -q "DKIM-Signature:" .mail/someone@testserver
@@ -40,11 +43,14 @@ dkimverify -txt .dkimcerts/dns.txt < .mail/someone@testserver
 tail -n +2 .mail/someone@testserver > .signed_content
 
 # Not authenticated: someone@testserver -> someone@testserver
-smtpc.py --server=localhost:1025 < .signed_content
+smtpc --addr=localhost:1025 \
+	--from=someone@testserver someone@testserver < .signed_content
 
 # Check that the signature fails on modified content.
 echo "Added content, invalid and not signed" >> .signed_content
-if smtpc.py --server=localhost:1025 < .signed_content 2> /dev/null; then
+if smtpc --addr=localhost:1025 \
+	--from=someone@testserver someone@testserver < .signed_content \
+	> /dev/null 2>&1 ; then
 	fail "DKIM verification succeeded on modified content"
 fi