tests: Fix signed/unsigned handling in pixdata_equal()
tests: Fix signed/unsigned handling in pixdata_equal()

The return values from the GdkPixbuf getters are signed, so assign them
to signed variables and then check that the returned values are
non-negative.

Coverity IDs: 1388535, 1388536, 1388537

https://bugzilla.gnome.org/show_bug.cgi?id=777374

GNOME/gdk-pixbuf,GNOME/gdk-pixbuf,GNOME/gdk-pixbuf,GNOME/gdk-pixbuf