Fix mail test

- Create an empty mailbox file in `/var/mail` so that `inotifywait` can
  wait on the right file and not just observe the directory.

e2e-tests/common.sh

@@ -12,8 +12,14 @@ delete_unix_user() { # name
 
 # send a mail and wait for delivery
 send_test_mail() { # email_address
+  USER=$(echo $1 | cut -f 1 -d '@')
+  # create an empty mailbox file
+  touch /var/mail/$USER
+  chown $USER:mail /var/mail/$USER
+  chmod 600 /var/mail/$USER
+
   echo "test mail" | sendmail $1
-  inotifywait -qq -t 3 -e close_write /var/mail
+  inotifywait -qq -t 3 -e close_write /var/mail/$USER
 }
 
 # test header