use tty6 for installedtest post_fail_hook
    
    Summary:
    tty2 is where wayland desktop sessions run. I think it makes
    sense to use a high tty for the post_fail_hook, so we know the
    lower ones can be used by the tests...
    
    Test Plan:
    Run a Workstation post-install test that fails, check
    the hook works.
    
    Reviewers: jskladan, garretraziel
    
    Reviewed By: garretraziel
    
    Subscribers: tflink
    
    Differential Revision: https://phab.qadevel.cloud.fedoraproject.org/D1003