Unicode characters are printed incorrectly with check string

[feverdreme]

When using Alcotest.check with Alcotest.string as its testable and there is a failure, alcotest prints the expected and received outputs incorrectly if they are Unicode characters.

Here is a minimum example:

open Alcotest

let foo () =
  check string "Dummy test" "∀" "∃"

let () =
  run "Test" [
    "Test", [
      test_case "test_case" `Quick foo;
    ];
  ]

Clearly the expected and received values are different, and here is the result:

File "test/dune", line 2, characters 7-22:
2 |  (name test_my_project)
           ^^^^^^^^^^^^^^^
Testing `Test'.
This run has ID `AT8IJ0GN'.

> [FAIL]        Test          0   test_case.

┌──────────────────────────────────────────────────────────────────────────────┐
│ [FAIL]        Test          0   test_case.                                   │
└──────────────────────────────────────────────────────────────────────────────┘
ASSERT Dummy test
FAIL Dummy test

   Expected: `"\226\136\128"'
   Received: `"\226\136\131"'

Raised at Alcotest_engine__Test.check in file "src/alcotest-engine/test.ml", line 200, characters 4-261
Called from Alcotest_engine__Core.Make.protect_test.(fun) in file "src/alcotest-engine/core.ml", line 181, characters 17-23
Called from Alcotest_engine__Monad.Identity.catch in file "src/alcotest-engine/monad.ml", line 24, characters 31-35

Logs saved to `~/Documents/alcotestbug/_build/default/test/_build/_tests/Test/Test.000.output'.
 ──────────────────────────────────────────────────────────────────────────────

Full test results in `~/Documents/alcotestbug/_build/default/test/_build/_tests/Test'.
1 failure! in 0.000s. 1 test run.

I tested this when using custom pretty printers and there was no such error.

open Alcotest

type t = {name: string}

let pp_person fmt person =
  Fmt.pf fmt "{ foo = %s }" person.name

let person_testable = testable pp_person (=)

(* Test the find_person function with a pattern match *)
let test_find_person () =
  check person_testable "Dummy test" {name = "∀"} {name = "∃"}

let () =
  run "Test" [
    "Test", [
      test_case "test_case" `Quick test_find_person;
    ];
  ]

File "test/dune", line 2, characters 7-22:
2 |  (name test_my_project)
           ^^^^^^^^^^^^^^^
Testing `Test'.
This run has ID `MJPDELRM'.

> [FAIL]        Test          0   test_case.

┌──────────────────────────────────────────────────────────────────────────────┐
│ [FAIL]        Test          0   test_case.                                   │
└──────────────────────────────────────────────────────────────────────────────┘
ASSERT Dummy test
FAIL Dummy test

   Expected: `{ foo = ∀ }'
   Received: `{ foo = ∃ }'

Raised at Alcotest_engine__Test.check in file "src/alcotest-engine/test.ml", line 200, characters 4-261
Called from Alcotest_engine__Core.Make.protect_test.(fun) in file "src/alcotest-engine/core.ml", line 181, characters 17-23
Called from Alcotest_engine__Monad.Identity.catch in file "src/alcotest-engine/monad.ml", line 24, characters 31-35

Logs saved to `~/Documents/alcotestbug/_build/default/test/_build/_tests/Test/Test.000.output'.
 ──────────────────────────────────────────────────────────────────────────────

Full test results in `~/Documents/alcotestbug/_build/default/test/_build/_tests/Test'.
1 failure! in 0.000s. 1 test run.