reftests: Fix an xfail to work

reftests: Fix an xfail to work

The background-image-multiple.ref.ui file uses
non-existing properties, which gives us a g_warning,
and the glib test framework insists on treating
warnings as fatal, so we end up doing exit(133),
which in turn makes the meson TAP parser ignore
its xfails.

Comment out the nonexisting properties, so we can
fail properly, and then in turn xfail properly.