xwayland: Add and hook up test script

It runs XTS via piglit on (non-rootless) Xwayland on weston using the
headless backend.

Xwayland might use glamor if enabled in the build, but we're making sure
it uses software rendering.

v2:
* Use weston-info to wait for weston to be ready, instead of just a
  fixed sleep. (Martin Peres)
v3:
* Build wayland 1.18 & weston 9.0 locally, since the packages in Debian
  buster are too old for current Xwayland.

Acked-by: Olivier Fourdan <ofourdan@redhat.com>
4 jobs for xwayland-test in 4 minutes and 20 seconds (queued for 1 second)
Status Job ID Name Coverage
  Docker Image
passed #5654680
debian-buster

00:00:10

 
  Build And Test
passed #5654681
autotools

00:04:09

passed #5654682
meson

00:01:02

passed #5654683
meson-noglamor

00:00:48