test: Add a test for DSN and null address deliveries

This patch adds a test for delivery status notifications and null address
deliveries, that check that chasquid can both receive and send DSNs.

To do this, we extend the mail_diff utility to support wildcards in the
comparisons, to skip over variable parts of the messages (like dates).

test/t-05-null_address/msmtprc

@@ -0,0 +1,13 @@
+account default
+
+host testserver
+port 1587
+
+tls on
+tls_trust_file config/domains/testserver/cert.pem
+
+from user@testserver
+
+auth on
+user user@testserver
+password secretpassword