Don't try bypass_1691487 for graphical base_selinux runs
    
    This just doesn't work when running the test on a graphical
    install because we're at a desktop when we try and do it, not
    a text console. So let's only try to do it if there is no
    DESKTOP. See e.g.:
    
    https://openqa.fedoraproject.org/tests/408484#step/base_selinux/2
    
    Signed-off-by: Adam Williamson <awilliam@redhat.com>