Workaround 'unused result' code defects in list_atomic.template

Workaround 'unused result' code defects in list_atomic.template

* tests/list_atomic.template: Include stdlib.h (needed for exit()
prototype).
* tests/list_atomic.template [AO_HAVE_XSIZE_loadXX]
(XSIZE_list_atomicXX): Cast AO_XSIZE_loadXX() result to void.
* tests/list_atomic.template [AO_HAVE_XSIZE_fetch_and_addXX]: Cast
AO_XSIZE_fetch_and_addXX() result to void.
* tests/list_atomic.template [AO_HAVE_XSIZE_fetch_and_add1XX]
(XSIZE_list_atomicXX): Cast AO_XSIZE_fetch_and_add1XX() result to void.
* tests/list_atomic.template [AO_HAVE_XSIZE_fetch_and_sub1XX]
(XSIZE_list_atomicXX): Cast AO_XSIZE_fetch_and_sub1XX() result to void.
* tests/list_atomic.template [AO_HAVE_test_and_setXX]
(XSIZE_list_atomicXX): Cast AO_test_and_setXX() result to void.
* tests/list_atomic.template [AO_HAVE_XSIZE_compare_and_swapXX]
(XSIZE_list_atomicXX): Call exit(1) if AO_XSIZE_compare_and_swapXX
failed.
* tests/list_atomic.template [AO_HAVE_XSIZE_fetch_compare_and_swapXX]
(XSIZE_list_atomicXX): Call exit(1) if AO_XSIZE_fetch_compare_and_swapXX
failed.