doc: add common.WPT to test README

PR-URL: #11127
Reviewed-By: Timothy Gu <timothygu99@gmail.com>
Reviewed-By: Joyee Cheung <joyeec9h3@gmail.com>
Reviewed-By: Michael Dawson <michael_dawson@ca.ibm.com>
Reviewed-By: James M Snell <jasnell@gmail.com>

test/README.md

@@ -379,3 +379,12 @@ The realpath of the 'tmp' directory.
 * return [<String>](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#String_type)
 
 Name of the temp directory used by tests.
+
+### WPT
+
+A port of parts of
+[W3C testharness.js](https://github.com/w3c/testharness.js) for testing the
+Node.js
+[WHATWG URL API](https://nodejs.org/api/url.html#url_the_whatwg_url_api)
+implementation with tests from
+[W3C Web Platform Tests](https://github.com/w3c/web-platform-tests).