xwayland: Disable the XTEST extension by default

Most features from the XTEST extension cannot work on Wayland, which can
be misleading for applications which would (optionally) rely on that
extension to implement some features.

Disable the XTEST extension by default to make it clear that XTEST is
not supposed to work on Xwayland, so that X11 clients are not tempted to
use it.

Using the standard command line option "+extension XTEST" can re-enable
it at runtime, if really needed.

Signed-off-by: Olivier Fourdan <ofourdan@redhat.com>
Reviewed-by: Peter Hutterer <peter.hutterer@who-t.net>
Acked-by: Karol Herbst <karolherbst@gmail.com>
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