[OCaml] Run tests twice, with ocamlc and ocamlopt (if available)

ocamlc and ocamlopt expose a distinct set of buildsystem bugs, e.g.
only ocamlc would detect -custom or -dllib-related bugs, and as all
buildbots will have ocamlopt, these bugs will stay hidden.

This change should add no more than 30 seconds of testing time.

llvm-svn: 221137