Allow gctest and thread_leak_test with zero NTHREADS

Allow gctest and thread_leak_test with zero NTHREADS

* tests/test.c [GC_PTHREADS || GC_WIN32_THREADS] (reverse_test_inner):
Do not call fork_a_thread() if NTHREADS is 0.
* tests/test.c (MAX_FINALIZED): Increase to be correct for the case of
NTHREADS is 0.
* tests/test.c [GC_PTHREADS] (main): Do not declare and do not use th,
i local variables if NTHREADS is 0.
* tests/thread_leak_test.c (NTHREADS): Do not define if already defined.
* tests/thread_leak_test.c (main):  Do not declare and do not use i,
code, t, thread_id local variables if NTHREADS is 0.