When tests are larger than the screen size and no compositing is
enabled, the window will be clipped to the screen size and all areas
outside of the screen have undefined contents.
To avoid this, we can use an offscreen window.
Checks that the size requests for labels are as they should be for
required and natural size given various combinations of wrap, ellipsize,
width-chars and max-width-chars.
See
http://mail.gnome.org/archives/gtk-devel-list/2011-April/msg00036.html
for the discussion spawning this test.
