Add a test that exposes a bug involving callbacks in multi-threaded code.
Add a test that exposes a bug involving callbacks in multi-threaded code.

whitequark/ocaml-ctypes,yallop/ocaml-ctypes,dsheets/ocaml-ctypes,ocamllabs/ocaml-ctypes,dsheets/ocaml-ctypes,whitequark/ocaml-ctypes,whitequark/ocaml-ctypes,ocamllabs/ocaml-ctypes,dsheets/ocaml-ctypes,yallop/ocaml-ctypes