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.