Fix gctest failure for Darwin if CPPCHECK is defined

Fix gctest failure for Darwin if CPPCHECK is defined
(fix commits c5d4ef9, 857bb8a)

* tests/test.c [TEST_FORK_WITHOUT_ATFORK] (NO_TEST_HANDLE_FORK):
Define even if CPPCHECK (i.e., CPPCHECK should not imply testing of
HANDLE_FORK).
* tests/test.c [GC_PTHREADS && CPPCHECK] (main): Remove duplicate check
for GC_PTHREADS.
* tests/test.c [GC_PTHREADS && CPPCHECK && NO_TEST_HANDLE_FORK] (main):
Call UNTESTED() for GC_atfork_child, GC_atfork_parent,
GC_atfork_prepare, GC_set_handle_fork, GC_start_mark_threads.