fix extra path separator in mkdir_p

[bbonf]

Closes #404

[dinosaure]

@MisterDA can you confirm that it's the right fix? Thanks.

[MisterDA]

@bbonf I can't reproduce the error. How did you encounter it? Could you add a failing test case perhaps?

[bbonf]

I'm not sure how to approach adding a test case for this, but here's how I reproduce it:

1. Installed ocaml on cygwin (windows 10 build 19045)
2. Created a new switch with ocaml-system.4.14.2
3. opam install dune alcotest
4. dune init project foo (in /tmp but I don't think it matters)
5. Added (libraries alcotest) to test/dune
6. Pasted this into test/test_foo.ml:

open Alcotest

let test () =
  (check @@ int) "just a test"
    0
    1

let () =
  run "Bar" [
    "test", [
      test_case "something" `Quick test
    ]
  ]

7. Run dune test

[MisterDA]

Installed ocaml on cygwin (windows 10 build 19045)

Do you mean that you're using the Cygwin port of OCaml (or the OCaml package from the Cygwin package archive)?
Or that you're using a Cygwin shell (and have the MinGW-w64 or MSVC ports of OCaml)?

[bbonf]

Installed the OCaml package from the Cygwin package archive.

(I think I tried and failed to use the mingw port but that's another story)