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>
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