testsuite/lib-threads: still working around Windows's exit bug

git-svn-id: http://caml.inria.fr/svn/ocaml/trunk@13693 f963ae5c-01c2-4b8c-9fe0-0dff7051ff02
PR#5710: add Mono support to the testsuite

git-svn-id: http://caml.inria.fr/svn/ocaml/trunk@13694 f963ae5c-01c2-4b8c-9fe0-0dff7051ff02