mdx_test: Lazily initialize OCaml toplevel

This changes the OCaml toplevel to be a lazy value, which is only
forced when needed to run tests in an OCaml block.

The motivation behind this change is to avoid stderr output from the
init process (e.g., errors while registering pretty-printers) spilling
into the test executable's output, which can trip up some build tools.

lib/test/mdx_test.ml

@@ -283,8 +283,9 @@ let run_exn ~non_deterministic ~silent_eval ~record_backtrace ~syntax ~silent
     match syntax with Some syntax -> Some syntax | None -> Syntax.infer ~file
   in
   let c =
-    Mdx_top.init ~verbose:(not silent_eval) ~silent ~verbose_findlib ~directives
-      ~packages ~predicates ()
+    lazy
+      (Mdx_top.init ~verbose:(not silent_eval) ~silent ~verbose_findlib ~directives
+         ~packages ~predicates ())
   in
   let preludes = preludes ~prelude ~prelude_str in
 
@@ -304,7 +305,7 @@ let run_exn ~non_deterministic ~silent_eval ~record_backtrace ~syntax ~silent
           let det () =
             assert (syntax <> Some Cram);
             Mdx_top.in_env env (fun () ->
-                eval_ocaml ~block:t ?syntax ?root c ppf t.contents errors)
+                eval_ocaml ~block:t ?syntax ?root (Lazy.force c) ppf t.contents errors)
           in
           with_non_det non_deterministic non_det ~command:print_block
             ~output:det ~det
@@ -337,7 +338,7 @@ let run_exn ~non_deterministic ~silent_eval ~record_backtrace ~syntax ~silent
                 (fun (phrase : Toplevel.t) ->
                   match
                     Mdx_top.in_env env (fun () ->
-                        eval_test ~block:t ?root c phrase.command)
+                        eval_test ~block:t ?root (Lazy.force c) phrase.command)
                   with
                   | Ok _ -> ()
                   | Error e ->
@@ -348,7 +349,7 @@ let run_exn ~non_deterministic ~silent_eval ~record_backtrace ~syntax ~silent
             ~det:(fun () ->
               assert (syntax <> Some Cram);
               Mdx_top.in_env env (fun () ->
-                  run_toplevel_tests ?syntax ?root c ppf phrases t))
+                  run_toplevel_tests ?syntax ?root (Lazy.force c) ppf phrases t))
     else print_block ()
   in
   let gen_corrected file_contents items =
@@ -357,7 +358,7 @@ let run_exn ~non_deterministic ~silent_eval ~record_backtrace ~syntax ~silent
     let buf = Buffer.create (String.length file_contents + 1024) in
     let ppf = Format.formatter_of_buffer buf in
     let envs = Document.envs items in
-    let eval lines () = eval_raw ?root c lines in
+    let eval lines () = eval_raw ?root (Lazy.force c) lines in
     let eval_in_env lines env = Mdx_top.in_env env (eval lines) in
     List.iter
       (function