Test store/CAS emulation explicitly

Test store/CAS emulation explicitly

* tests/Makefile.am (test_atomic_generalized_CPPFLAGS): Add
-D AO_TEST_EMULATION.
* tests/test_atomic.c [(!_MSC_VER && !__MINGW32__ && !__BORLANDC__
|| AO_USE_NO_SIGNALS || AO_USE_WIN32_PTHREADS) && AO_TEST_EMULATION]
(AO_store_full_emulation, AO_fetch_compare_and_swap_emulation): Declare
prototype.
* tests/test_atomic.c [(!_MSC_VER && !__MINGW32__ && !__BORLANDC__
|| AO_USE_NO_SIGNALS || AO_USE_WIN32_PTHREADS) && AO_TEST_EMULATION
&& AO_HAVE_double_t] (AO_compare_double_and_swap_double_emulation):
Likewise.
* tests/test_atomic.c (test_atomic_emulation): New function (or a macro
defined to empty if AO_*_emulation primitives are unavailable).
* tests/test_atomic.c (main): Call test_atomic_emulation().