fixed test to test what happens when we close a file descriptor on which another thread is blocked reading

git-svn-id: http://caml.inria.fr/svn/ocaml/trunk@13675 f963ae5c-01c2-4b8c-9fe0-0dff7051ff02
testsuite/lib-threads: stop all threads before exiting

git-svn-id: http://caml.inria.fr/svn/ocaml/trunk@13676 f963ae5c-01c2-4b8c-9fe0-0dff7051ff02
testsuite/backtrace: fix problem on cygwin with leftover executable file

git-svn-id: http://caml.inria.fr/svn/ocaml/trunk@13677 f963ae5c-01c2-4b8c-9fe0-0dff7051ff02