desktop_terminal: assert we actually see terminal app
    
    ...Apparently a wait_still_screen 5 isn't always enough on
    aarch64:
    https://openqa.stg.fedoraproject.org/tests/963387#step/desktop_terminal/2
    so, this. Note that konsole_runs has apps_run_terminal tag, so
    this won't break the test on KDE.
    
    Signed-off-by: Adam Williamson <awilliam@redhat.com>