CRM-13891 - CiviMail - Use brackets around civicrm draft in subject line.

This seems to be the convention these days.

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* CRM-13891: remove word "test" from the subject line of test email
  http://issues.civicrm.org/jira/browse/CRM-13891

CRM/Mailing/BAO/MailingJob.php

@@ -518,7 +518,7 @@ VALUES (%1, %2, %3, %4, %5, %6, %7)
     $fields = array();
 
     if (!empty($testParams)) {
-      $mailing->subject = ts('CiviMail Draft -') . ' ' . $mailing->subject;
+      $mailing->subject = ts('[CiviMail Draft]') . ' ' . $mailing->subject;
     }
 
     CRM_Mailing_BAO_Mailing::tokenReplace($mailing);