add a desktop_update_graphical test
    
    Summary:
    Very similar to the CLI update test, but using the desktops'
    update applications. This is based off the CLI update test
    branch as it uses the shared functions that branch introduced.
    We do not use the fake update packages, as they don't really
    do anything useful for these tests; for dnf they can help us
    distinguish between issues with the dnf mechanism and issues
    with the repos, but we can't really tell that in the graphical
    case. So we only use the python3-kickstart package here.
    
    Test Plan:
    Run the test on both KDE and GNOME and ensure it
    performs as intended. I've been testing it on staging, so you
    can see it there.
    
    Reviewers: jskladan, garretraziel
    
    Reviewed By: garretraziel
    
    Subscribers: tflink
    
    Differential Revision: https://phab.qadevel.cloud.fedoraproject.org/D1010