Fix env test to check for proper exception

The test didn't check for what it actually wanted to test. The contained
code did actually throw, however that was only the case, because jsdom
was undefined, instead of a thrown error by jsdom.env.
Add more clearity to tests run by the w3c suite

Pages where script errors occur will now result in a failed test and
successful assertions will also be counted.
localName should not be null for ELEMENT_NODE
Add document.implementation.createHTMLDocument()
Add a note about how to update the web-platform-tests submodules
Removed a lot of unused requires
Refactor DOMWindow into its own file
Rename DOMWindow to Window
Moved lots of properties to level1

This is to reduce monkey patching these properties afterwards.

If we need an object, we should define it in a single file, not
distribute its code over 5.
Move nodeName to Element prototype
Fix casing issues in level1 tests
Remove horrible tests from a bygone era
Remove tests which fail due to DTD checks
Remove even more tests testing random attributes
Fix casing issues in level2/core
Remove "default attribute" related tests from level2/core
Remove level1/svg tests!

They were literally the exact same tests as the level1/core tests, except with a different xmlns attribute on the document root.
Move parentWindow setter to level1
Removed all traces of levels

The only thing remaining is the defaultLevel export. I left that in for
now in order to allow someone to overwrite internals without having to
resort to require()ing deep into our internal structure.
Only apply the selector functions once