Replace shelling out to diff by helper program

This should be more portable since we don't depend on `diff` being
installed or have an exit code of 1 when the files differ.

Author: Leonidas-from-XIV

test/bin/mdx-dune-gen/misc/non-deterministic/different.ml

@@ -0,0 +1,38 @@
+(* small helper to determine whether two files differ *)
+
+let main () =
+  let first = Sys.argv.(1) |> open_in in
+  let second = Sys.argv.(2) |> open_in in
+  let rec loop () =
+    match input_line first with
+    | first_line -> (
+        match input_line second with
+        | second_line ->
+            match String.equal first_line second_line with
+            | true -> loop ()
+            | false ->
+                (* we found a difference between the lines *)
+                exit 0
+        | exception End_of_file ->
+            (* the second file ended before the first *)
+            exit 0
+    )
+    | exception End_of_file ->
+        (* the first file ended first *)
+        match input_line second with
+        | _ ->
+            (* the second file continues: a difference *)
+            exit 1
+        | exception End_of_file ->
+            (* the second file ended too *)
+            ()
+  in
+  loop ();
+  close_in first;
+  close_in second;
+  (* we didn't find a difference, exit with a failure code *)
+  prerr_endline "The files appear to be identical";
+  exit 1
+
+let () =
+  main ()

test/bin/mdx-dune-gen/misc/non-deterministic/dune

@@ -30,11 +30,12 @@
 
 ;; make sure the non-deterministic is different from the deterministic
 
+(executable
+ (name different)
+ (modules different))
+
 (rule
  (alias runtest)
  (action
-  (with-accepted-exit-codes
-   1
-   (ignore-stdout
-    (run diff %{dep:dune-mdx-nondeterministic.expected}
-      %{dep:dune-mdx-nondeterministic.nondeterministic})))))
+  (run ./different.exe %{dep:dune-mdx-nondeterministic.expected}
+    %{dep:dune-mdx-nondeterministic.nondeterministic})))