test: add TODO to encoding tests that can be replaced with WPT

PR-URL: #25155
Reviewed-By: James M Snell <jasnell@gmail.com>

Author: joyeecheung

test/parallel/test-whatwg-encoding-surrogates-utf8.js

@@ -1,6 +1,7 @@
 'use strict';
 
 // From: https://github.com/w3c/web-platform-tests/blob/fa9436d12c/encoding/api-surrogates-utf8.html
+// TODO(joyeecheung): replace this with WPT
 
 require('../common');