test: fix screenshot API impl
    
    When redoing the website we deleted the libvirtLogo.png file
    not remembering that the test driver screenshot API impl
    relied on it.
    
    Rather than having the test driver use the logo as a side
    effect, give it its own dedicated image to use. This is
    installed in /usr/share/libvirt/test-screenshot.png and
    is taken from a NeXT Cube running WorldWideWeb[1]. The
    very first web browser in existance, running on the
    hardware it was originally written on.
    
    [1] https://en.wikipedia.org/wiki/WorldWideWeb
    
    Signed-off-by: Daniel P. Berrange <berrange@redhat.com>