test: Only run X11-specific code when using X11
test: Only run X11-specific code when using X11

The X11-specific windowing checks were hidden behind an #ifdef, however
if the tests were run under Wayland, they would execute uncondionally
and cause assertion failures.  Fix this by also hiding them behind a
check that the current backend is indeed X11.

Signed-off-by: Daniel Stone <3d0f3b9ddcacec30c4008c5e030e6c13a478cb4f@fooishbar.org>

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