IPA-EPN: Add tests for --mail-test option
    
    Test sending a default template email to the smtp_admin user.
    
    Test that --mail-test and --dry-run cannot be used together.
    
    https://pagure.io/freeipa/issue/3687
    Signed-off-by: Rob Crittenden <rcritten@redhat.com>
    Reviewed-By: Francois Cami <fcami@redhat.com>