tests/w3c seems to be populated by the SHA referenced in package.json['web-platform-tests'].repo. It's currently pointing at a really old SHA (from 2015). When I try to update it, I receive this error:

Running "pretest" task
Unhandled rejection Error: EISDIR: illegal operation on a directory, open '~/Projects/PEP/tests/w3c/resources'
    at Error (native)

If I kill the process and run it again, it goes further, but most of the tests fail:

NoSuchElement: [POST http://localhost:4444/wd/hub/session/f93260d5-52de-439a-bf86-bca9471abfa0/element / {"using":"id","value":"target0"}] no such element: Unable to locate element: {"method":"id","selector":"target0"}

or

ScriptTimeout: [POST http://localhost:4444/wd/hub/session/f93260d5-52de-439a-bf86-bca9471abfa0/execute_async / {"script":"return (function (poller, args, timeout, pollInterval, done) {\n\t\t\t\t\t\t/* jshint evil:true */\n\t\t\t\t\t\tpoller = new Function(poller);\n\n\t\t\t\t\t\tvar endTime = Number(new Date()) + timeout;\n\n\t\t\t\t\t\t(function poll() {\n\t\t\t\t\t\t\tvar result = poller.apply(this, args);\n\n\t\t\t\t\t\t\t/*jshint evil:true */\n\t\t\t\t\t\t\tif (result != null) {\n\t\t\t\t\t\t\t\tdone(result);\n\t\t\t\t\t\t\t}\n\t\t\t\t\t\t\telse if (Number(new Date()) < endTime) {\n\t\t\t\t\t\t\t\tsetTimeout(poll, pollInterval);\n\t\t\t\t\t\t\t}\n\t\t\t\t\t\t\telse {\n\t\t\t\t\t\t\t\tdone(null);\n\t\t\t\t\t\t\t}\n\t\t\t\t\t\t})();\n\t\t\t\t\t}).apply(this, arguments);","args":["return (function () {\n\t\t\t\t\treturn window.w3cTests;\n\t\t\t\t}).apply(this, arguments);",[],1000,67]}] asynchronous script timeout: result was not received in 1 seconds

My current strategy is to look at the git history (back 20 pages, back 10 pages, etc.) to figure out which commit broke the PEP tests. It's effectively git bisect, but across repos.