Use create_process instead of execvp

On Windows, execvp creates a separate process, which can confuse dune
when it is waiting for the mdx process. This causes it to miss that the
output file has been created. This affects mdx's test suite and
possibly users of the mdx stanza. Instead we create a process and
immediately wait for it in the parent.

See realworldocaml#295, ocaml/dune#3849.

bin/test.ml

@@ -41,7 +41,12 @@ let run (`Setup ()) _ _ _ _ _ _ _ _ _ _ =
   let argv = Array.sub Sys.argv 1 (Array.length Sys.argv - 1) in
   argv.(0) <- cmd;
   Log.debug (fun l -> l "executing %a" Fmt.(Dump.array string) argv);
-  Unix.execvp cmd argv
+  let pid = Unix.create_process cmd argv Unix.stdin Unix.stdout Unix.stderr in
+  let _pid, status = Unix.waitpid [] pid in
+  let exit_code =
+    match status with WEXITED n -> n | WSIGNALED _ -> 1 | WSTOPPED _ -> 1
+  in
+  exit exit_code
 
 open Cmdliner