URL spec test coverage report #3018

FYI @annevk @Sebmaster @jasnell.

I managed to cobble this together:

• URL state machine coverage report: ~99%
• URL object coverage report: ~94%

Areas of lacking coverage are:

• some origin serialization stuff (some purposefully untested)
• The href setter
• One early-return case in the hash setter
• One "else" branch in IPv4 parsing