util: use private symbols in JS land directly

Co-authored-by: Antoine du Hamel <duhamelantoine1995@gmail.com>

test/parallel/test-util-internal.js

@@ -14,10 +14,10 @@ const {
 
 const obj = {};
 assert.strictEqual(obj[arrow_message_private_symbol], undefined);
-assert.deepStrictEqual(Reflect.ownKeys(obj), []);
 
 obj[arrow_message_private_symbol] = 'bar';
 assert.strictEqual(obj[arrow_message_private_symbol], 'bar');
+assert.deepStrictEqual(Reflect.ownKeys(obj), []);
 
 let arrowMessage;