Fix broken tests

* Put FETCHMAIL_POLL env variable in Dockerfile to handle case where
  user does not specify it in their docker-compose.yml

Dockerfile

@@ -5,6 +5,7 @@ ENV DEBIAN_FRONTEND noninteractive
 ENV VIRUSMAILS_DELETE_DELAY=7
 ENV ONE_DIR=0
 ENV ENABLE_POSTGREY=0
+ENV FETCHMAIL_POLL=300
 ENV POSTGREY_DELAY=300
 ENV POSTGREY_MAX_AGE=35
 ENV POSTGREY_TEXT="Delayed by postgrey"