last forgotten test

git-svn-id: http://caml.inria.fr/svn/ocaml/version/4.02@14887 f963ae5c-01c2-4b8c-9fe0-0dff7051ff02
PR#6332: Unix.open_process fails to pass empty arguments under Windows

git-svn-id: http://caml.inria.fr/svn/ocaml/version/4.02@14888 f963ae5c-01c2-4b8c-9fe0-0dff7051ff02