tests: port large-fd POSIX spawn tests to OS X

tests: port large-fd POSIX spawn tests to OS X

Problem reported by Daiki Ueno in
<http://lists.gnu.org/archive/html/bug-gnulib/2013-06/msg00031.html>.
* tests/test-posix_spawn_file_actions_addclose.c:
* tests/test-posix_spawn_file_actions_adddup2.c:
* tests/test-posix_spawn_file_actions_addopen.c:
Include <limits.h>, for OPEN_MAX, if available.
(big_fd): New static function.
(main): Use it.