tests: Add test for wl_list_length

list-test.c did not cover wl_list_length, 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>
Reviewed-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>