[ticket/16180] Allow adding message to test email

PHPBB3-16180

phpBB/language/en/email/test.txt

@@ -4,6 +4,10 @@ Hello {USERNAME},
 
 Congratulations. If you received this email, phpBB is correctly configured to send emails.
 
+<!-- IF MESSAGE -->
+{MESSAGE}
+
+<!-- ENDIF -->
 In case you require assistance, please visit the phpBB support forums - https://www.phpbb.com/community/
 
 {EMAIL_SIG}