test: add test for promisify customPromisifyArgs

PR-URL: nodejs#12442
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Benjamin Gruenbaum <benjamingr@gmail.com>
Reviewed-By: Matteo Collina <matteo.collina@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Myles Borins <myles.borins@gmail.com>
Reviewed-By: Evan Lucas <evanlucas@me.com>
Reviewed-By: William Kapke <william.kapke@gmail.com>
Reviewed-By: Timothy Gu <timothygu99@gmail.com>
Reviewed-By: Teddy Katz <teddy.katz@gmail.com>

test/parallel/test-util-promisify.js

@@ -1,9 +1,11 @@
 'use strict';
+// Flags: --expose-internals
 const common = require('../common');
 const assert = require('assert');
 const fs = require('fs');
 const vm = require('vm');
 const { promisify } = require('util');
+const { customPromisifyArgs } = require('internal/util');
 
 common.crashOnUnhandledRejection();
 
@@ -42,6 +44,21 @@ const stat = promisify(fs.stat);
                                 'be a function');
 }
 
+{
+  const firstValue = 5;
+  const secondValue = 17;
+
+  function fn(callback) {
+    callback(null, firstValue, secondValue);
+  }
+
+  fn[customPromisifyArgs] = ['first', 'second'];
+
+  promisify(fn)().then(common.mustCall((obj) => {
+    assert.deepStrictEqual(obj, {first: firstValue, second: secondValue});
+  }));
+}
+
 {
   const fn = vm.runInNewContext('(function() {})');
   assert.notStrictEqual(Object.getPrototypeOf(promisify(fn)),