ocamltest Win32: fix timeout handling
ocamltest Win32: fix timeout handling

The original code caused the timeout duration to drop to 0 at some
of the spurious wake-ups, hence the command would time out early.

The new code uses GetTickCount64() instead of GetTickCount() to avoid
issues with GetTickCount() overflowing and wrapping around.
(As suggested by David Allsopp.)

gerdstolpmann/ocaml,gerdstolpmann/ocaml,gerdstolpmann/ocaml,gerdstolpmann/ocaml,gerdstolpmann/ocaml