Disable prerelease text check in disk_guided_empty
    
    This is causing all kinds of trouble, because when the test is
    run on the Server DVD - with the 'orange to blue' gradient - the
    prerelease note is dark red text on a dark blue background.
    os-autoinst actually reduces the color depth of images/needles
    and greyscales them before performing the match...but for this
    dark red text on dark blue background, the result seems to be
    that the text and background come out *the same grey*, so *any*
    text will match the needle (even if it's completely different
    text), as will *no text at all*. I've tried finessing around
    this a few times but it just keeps happening, so for now I'm
    just disabling the pre-release text check at this point. We still
    have the check during _boot_to_anaconda, when the text appears
    on a *grey* background and so isn't a problem. I'm not removing
    the needles yet, until we hear back from upstream:
    
    https://progress.opensuse.org/issues/56822
    
    Signed-off-by: Adam Williamson <awilliam@redhat.com>