Tests mode-dependent foreign stubs.

  $ dune exec ./stubs_exe.exe
  Byte (0) or native (1) ? 1

  $ dune exec ./stubs_exe.bc.exe
  Byte (0) or native (1) ? 0

  $ dune exec ./stubs_lib.exe
  Byte (0) or native (1) ? 42

  $ dune exec ./stubs_lib.bc.exe
  Byte (0) or native (1) ? 0
