ocamlPackages.paf, ocamlPackages.paf-cohttp: add local networking for test

in test we can read this:
[exception] Unix.Unix_error(Unix.EPERM, "bind", "")
            Raised by primitive operation at Tcpv4v6_socket.listen.(fun) in file "src/stack-unix/tcpv4v6_socket.ml", line 149, characters 6-50

pkgs/development/ocaml-modules/paf/cohttp.nix

@@ -47,6 +47,8 @@ buildDunePackage {
     astring
   ];
 
+  __darwinAllowLocalNetworking = true;
+
   meta = paf.meta // {
     description = "A CoHTTP client with its HTTP/AF implementation";
   };

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

@@ -61,6 +61,8 @@ buildDunePackage rec {
     alcotest-lwt
   ];
 
+  __darwinAllowLocalNetworking = true;
+
   meta = {
     description = "HTTP/AF and MirageOS";
     license = lib.licenses.mit;