Move wptrunner testharness implementation into a single browsing context

[jgraham]

We have some tests like /html/browsers/the-windowproxy-exotic-object/windowproxy-prototype-setting-goes-cross-origin-domain.sub.html that alter the browser window in such a way that window.opener appears to be cross-origin. These tests end up throwing a security error. To fix this we probably need to put all the harness code in a single browsing context, mostly using testharnessreport.js to ensure that there isn't a race during load.