Use same macro (NTHREADS) across all tests to specify number of threads

Use same macro (NTHREADS) across all tests to specify number of threads
(code refactoring)

* tests/disclaim_test.c [GC_PTHREADS] (THREAD_CNT): Rename to NTHREADS;
do not define if already defined.
* tests/disclaim_test.c [!GC_PTHREADS] (THREAD_CNT): Rename to
NTHREADS; undefine it first.
* tests/disclaim_test.c (MUTATE_CNT, main): Rename THREAD_CNT to
NTHREADS.
* tests/subthread_create.c [!NTHREADS] (INITIAL_THREAD_COUNT): Rename
to NTHREADS; add comment.
* tests/subthread_create.c (main): Rename INITIAL_THREAD_COUNT to
NTHREADS.
* tests/threadkey_test.c (LIMIT, main): Rename LIMIT to NTHREADS.