package/spice: disable tests

By disabling tests, we'll remove the optional gdk-pixbuf dependency

Fixes:
 - http://autobuild.buildroot.org/results/96c786f85d35f33508e9c71778043d16b87f72cd



Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>