[test] assert_return should not assume it can stringify ref.array values

[Ms2ger]

This fails in array_new_elem.wast.js, for example:

// array_new_elem.wast:18
assert_return(() => call($1, "array-new-elem", [0, 0]), "array_new_elem.wast:18", "ref.array");

#9 Test that a WebAssembly code returns a specific result (array_new_elem.wast:18)

can't convert actual[i] to string

assert_return/chain</<@http://web-platform.test:8000/wasm/core/js/harness/async_index.js:338:92
Test.prototype.step@http://web-platform.test:8000/resources/testharness.js:2869:25
test@http://web-platform.test:8000/resources/testharness.js:633:30
uniqueTest@http://web-platform.test:8000/wasm/core/js/harness/async_index.js:27:7
assert_return/chain<@http://web-platform.test:8000/wasm/core/js/harness/async_index.js:310:19

with async_index.js:

[332]              case "ref.any":
[333]              case "ref.eq":
[334]              case "ref.struct":
[335]              case "ref.array":
[336]                // For now, JS can't distinguish exported Wasm GC values,
[337]                // so we only test for object.
[338]                assert_true(typeof actual[i] === "object", `expected Wasm GC object, got ${actual[i]}`);
[339]                return;