Make test mail only tell that it works when PHPMailer is actually enabled

The test email gets through with a message saying PHPMailer is configured correctly even if the "Use PHPMailer..." checkbox is not selected. In this case, the PHPMailer was not even used (which you can tell from the email headers), which suggests that this email isn't really testing whether PHPMailer works. I'm not sure if this is intended behaviour, but perhaps the test email should use PHPMailer even if the checkbox isn't selected.