xwayland/test: Don't catch errors in run-piglit.sh

The xwayland-piglit.sh script spawns weston, runs run-piglit.sh and
finally kills weston.

However, this whole script is running with “-e” meaning that any error
will cause the script to exit immediately.

As a result, if run-piglit.sh exits with a non-zero code such as 77 for
skipping the test, the script will exit prematurely leaving weston
running, and meson will simply wait until the timeout kicks in, and
fail eventually instead of skipping the test as it should.

Fix this by removing the option to exit immediately prior to spawn the
script run-piglit.sh.

Closes: xorg/xserver#1204
Suggested-by: Michel Dänzer <mdaenzer@redhat.com>
Signed-off-by: Olivier Fourdan <ofourdan@redhat.com>
Reviewed-by: Michel Dänzer <mdaenzer@redhat.com>
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