ocamlPackages.mdx: fix Darwin build

The unix set perm test fails on Darwin, so disable tests as I do not
believe there is a way to disable specific tests in OCaml.

Signed-off-by: Ethan Carter Edwards <ethan@ethancedwards.com>

pkgs/development/ocaml-modules/mdx/default.nix

@@ -1,11 +1,13 @@
 {
   lib,
+  stdenv,
   fetchurl,
   buildDunePackage,
   ocaml,
   findlib,
   alcotest,
   astring,
+  cmdliner,
   cppo,
   fmt,
   logs,
@@ -34,6 +36,7 @@ buildDunePackage rec {
     fmt
     logs
     csexp
+    cmdliner
     ocaml-version
     camlp-streams
     re
@@ -44,7 +47,7 @@ buildDunePackage rec {
     lwt
   ];
 
-  doCheck = true;
+  doCheck = !stdenv.hostPlatform.isDarwin;
 
   outputs = [
     "bin"