chore(ci): integrate mailhog into test compose file

Author: jeboehm

.ci/matrix/relayhost/test.sh

@@ -1,6 +1,9 @@
 version: '3.5'
 
 services:
+  mailhog:
+    image: mailhog/mailhog:v1.0.1
+
   test:
     build: ./test
     volumes:
@@ -11,6 +14,8 @@ services:
       # For development
       - ./test/rootfs/usr/share/tests:/usr/share/tests:ro
     env_file: .env
+    depends_on:
+      - mailhog
 
   # For development
   # web: