Eliminate data race in cons() of test_malloc

Eliminate data race in cons() of test_malloc
(fix commit c058d9d)

The data race has not affected the functionality of the test but this
commit allows to avoid no_sanitize attribute for the function.

* tests/test_malloc.c (cons): Remove AO_ATTR_NO_SANITIZE_THREAD
attribute; Do not reset extra (use "%" operator to get my_extra value).
* tests/test_malloc.c [AO_HAVE_fetch_and_add1] (cons): Change type of
extra to AO_t, add volatile qualifier; use AO_fetch_and_add1 instead of
"++" operator to update extra.