Fix data race in a list referenced by A.aa (gctest)

Fix data race in a list referenced by A.aa (gctest)
(Cherry-pick commits 8f5746e, 0f5a88d from 'master' branch.)

* tests/test.c [!AO_HAVE_load] (AO_load): Remove macro.
* tests/test.c [!AO_HAVE_store] (AO_store): Likewise.
* tests/test.c [!AO_HAVE_load_acquire] (AO_load_acquire): New static
function (which uses FINALIZER_LOCK/FINALIZER_UNLOCK).
* tests/test.c [!AO_HAVE_store_release] (AO_store_release): Likewise.
* tests/test.c [!AO_HAVE_fetch_and_add1] (AO_fetch_and_add1): Add
comment.
* tests/test.c (a_set): Use AO_store_release() instead of AO_store().
* tests/test.c (a_get): Use AO_load_acquire() instead of AO_load().
* tests/test.c (reverse_test_inner): Improve comment about thread
safety of a_set(reverse(reverse(a_get()))).