conformance: Only check for DISPLAY on X11 windowing backend
conformance: Only check for DISPLAY on X11 windowing backend

Fixes: https://bugzilla.gnome.org/show_bug.cgi?id=693741

GNOME/clutter,GNOME/clutter,Distrotech/clutter,djdeath/clutter,jigpu/clutter,collects/clutter,heysion/clutter-clone,jigpu/clutter,Distrotech/clutter,heysion/clutter-clone,djdeath/clutter,djdeath/clutter,djdeath/clutter,GNOME/clutter,heysion/clutter-clone,djdeath/clutter-android,GNOME/clutter,djdeath/clutter-android,ebassi/clutter,collects/clutter,djdeath/clutter-android,jigpu/clutter,Distrotech/clutter,djdeath/clutter-android,Distrotech/clutter,GNOME/clutter,collects/clutter,heysion/clutter-clone,jigpu/clutter,ebassi/clutter,collects/clutter,Distrotech/clutter,collects/clutter,ebassi/clutter,ebassi/clutter,heysion/clutter-clone,djdeath/clutter-android,jigpu/clutter,Distrotech/clutter,djdeath/clutter,jigpu/clutter,djdeath/clutter,ebassi/clutter