tests: Test wl_message_count_arrays

message-test.c did not cover wl_message_count_arrays, so add one test that
specifically tests this method. Note that this exposes wl_message_count_arrays
in a private header (wayland-private.h), and removes the `static` modifier of
the implementation.

Signed-off-by: Yong Bakos <ybakos@humanoriented.com>
Reviewed-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>