array-test: Include wayland-util.h and simplify init test

Include wayland-util.h in addition to wayland-private.h, to be more explicit
about where wl_array is defined.

Remove the useless repeated testing of wl_array_init, because if it fails once
out of thousands of iterations we're all doomed anyway.

Signed-off-by: Yong Bakos <ybakos@humanoriented.com>
Reviewed-by: Eric Engestrom <eric.engestrom@imgtec.com>
[Pekka: add the memset]
Signed-off-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>