tests: Test wl_array_release

array-test.c did not cover wl_array_release, so add one test that specifically
tests this method.

Signed-off-by: Yong Bakos <ybakos@humanoriented.com>
Reviewed-by: Eric Engestrom <eric.engestrom@imgtec.com>
[Pekka: do not overwrite array.data]
Signed-off-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>