fixed subject for test mail

src/main/java/de/srsoftware/widerhall/data/MailingList.java

@@ -684,7 +684,7 @@ public class MailingList implements MessageHandler, ProblemListener {
      * @throws UnsupportedEncodingException
      */
     public void test(User user) throws MessagingException, UnsupportedEncodingException {
-        var subject = t("{}: test mail",name());
+        var subject = t("[{}]: test mail",name());
         var text = "If you received this mail, the SMTP settings of your mailing list are correct.";
         smtp.login().send(email(),name(),user.email(),subject,text);
     }