test: mail hostname set to non-existent domain

Don't use a legigimate domain. as this is for testing only, set to null

MR !6

test/docker-compose.yml

@@ -83,7 +83,7 @@ services:
     restart: unless-stopped
     cpus: 2
     mem_limit: 512MB
-    hostname: test.example.org
+    hostname: test.nodomain.org
     volumes:
       - mail_store:/srv/mail:rw
       - mail_ssl:/certs