defaultvalue test: ignore show-desktop

defaultvalue test: ignore show-desktop

Ignore the "show-desktop" property on GtkPlacesSidebar for the
defaultvalue test.

Currently, "make check" is passing because it runs the test under a xvfb
with no XSETTINGS provider, so we see the Gtk default value.  No matter
what we set the default value to in Gtk, however, there will be some
desktop environment in which someone running the installed test outside
of an xvfb will get the wrong result.  Best to ignore it.

https://bugzilla.gnome.org/show_bug.cgi?id=712302