add a desktop notifications test
    
    Summary:
    this more or less covers desktop_error_checks and desktop_
    update_notification, though it can't really distinguish
    between them easily. All we know is that if both the live and
    postinstall versions of this test pass, both of those tests
    pass. Any fails will have to be investigated manually.
    
    Test Plan:
    Run the tests for both KDE and Workstation, see
    what happens. Workstation will fail for F25 and Rawhide at
    present, due to SELinux/abrt notifications.
    
    Reviewers: jskladan, garretraziel
    
    Reviewed By: garretraziel
    
    Subscribers: tflink
    
    Differential Revision: https://phab.qadevel.cloud.fedoraproject.org/D1004