Allow to alter DEFAULT/MAX_NTHREADS values in test_malloc/stack

Allow to alter DEFAULT/MAX_NTHREADS values in test_malloc/stack
(code refactoring)

* tests/run_parallel.h [!MAX_NTHREADS] (MAX_NTHREADS): Define (move
from test_malloc.c).
* tests/run_parallel.h [USE_PTHREADS || USE_VXTHREADS
|| USE_WINTHREADS] (run_parallel): Replace 100 with MAX_NTHREADS.
* tests/test_malloc.c (main): Add assertion that DEFAULT_NTHREADS
is not greater than MAX_NTHREADS.
* tests/test_stack.c (main): Likewise.
* tests/test_stack.c [!DEFAULT_NTHREADS] (DEFAULT_NTHREADS): Define
(to 4).
* tests/test_stack.c (main): Use DEFAULT_NTHREADS (instead of 4).