Use atomic allocation for leafs in reverse_test (gctest)

Use atomic allocation for leafs in reverse_test (gctest)

* tests/test.c [NO_CONS_ATOMIC_LEAF] (small_cons_leaf): Redirect to
small_cons.
* tests/test.c [!NO_CONS_ATOMIC_LEAF] (small_cons_leaf): Define function
(which uses GC_MALLOC_ATOMIC).
* tests/test.c (small_cons_uncollectable, uncollectable_ints): Replace
small_cons(INT_TO_SEXPR(low),nil) to small_cons_leaf(low).