Warn the user when testing outbound mail configuration if CIVICRM_MAIL_LOG is set

When the environment variable CIVICRM_MAIL_LOG is set CiviCRM won't try to send mail via any of the methods, but it will return success indicating that it did send mail.  Normally that's fine, except when you forget you've got that configured (or didn't realise - buildkit does it by default).  With this patch, when testing outbound email settings you will get a warning message if CIVICRM_MAIL_LOG is set so you don't scratch your head for ages wondering why email is not being sent out!