tests: improve check for display cache.
tests: improve check for display cache.

Improve check for display cache infrastructure. In particular, for X11 and
GLX backends, we need to make sure that we can create a GstVaapiDisplayX11
from another GstVaapiDisplayGLX, i.e. underlying X11 and VA displays can be
shared. Besides, allocating a GstVaapiDisplayGLX while a GstVaapiDisplayX11
already exists will have to generate different VA displays.

gbeauchesne/gstreamer-vaapi,01org/iotg-lin-gfx-gstreamer-vaapi,GStreamer/gstreamer-vaapi,GStreamer/gstreamer-vaapi,ceyusa/gstreamer-vaapi,gbeauchesne/gstreamer-vaapi,CapOM/gstreamer-vaapi,01org/iotg-lin-gfx-gstreamer-vaapi,CapOM/gstreamer-vaapi,ceyusa/gstreamer-vaapi,01org/iotg-lin-gfx-gstreamer-vaapi,CapOM/gstreamer-vaapi,gbeauchesne/gstreamer-vaapi