Add WIP tests for HTML's resolving URLs
Use add_cleanup
Test the ping attribute, plus some minor fixes
Test meta refresh
Test loading of nested browsing contests
Add spec link annotations
Test loading CSS with <link>
Test <script src>
Test loading images
Test loading video
Also test <source>
Test <track src>. (This test seems to expose a garbage collection bug in presto/blink/gecko.)
Test form submission
Test <base href>. Use blank.py instead of about:blank since the latter uses utf-8. Specify encoding in resource.py for types that would inherit.
Test microdata itemValue
Test Worker/SharedWorker constructors
Test EventSource constructor
Test XHR. Test importScripts/Worker/SharedWorker in dedicated workers and shared workers.
Test WebSocket() and WebSocket#url
Move helper files into resources/