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/content

@@ -0,0 +1,5 @@
+From: Mailer daemon <somewhere@horns.com>
+Subject: I've come to haunt you
+
+Muahahahaha
+