@lruzicka added this to the KDE app tests in 71c4e273, but there
was no need for a new needle as I'd already done the same thing
in the desktop updates tests; let's just use the same needle.
Signed-off-by: Adam Williamson <awilliam@redhat.com>
Since the KDE menus have transparency set, any time the wallpaper
changes the menus will look different, and the app tests may
fail. This sets the desktop wallpaper to black at the start of
the test suite to avoid this problem.
