Add URL IDL tests & drop stale (untested) URL IDLs

This change:

  [x] adds URL & URLUtils* IDL tests (up-to-date with current URL spec)
  [x] drops untested URL & URLUtils* IDLs from html/dom/interfaces.html
  [x] replaces old untested html/dom/interfaces.html URLUtils* IDLs w/ stubs

(Nothing in html/dom/interfaces.html references the URL interface anyway,
so there's no reason to include it there. And the URLUtils* IDLs that were
in there were out-of-date with the current URL spec, and regardless there's
no need to include the actual whole interfaces for them there, so instead
let's just put (untested) stubs for them there.)
Drop stray class=untested vestige in URL idl test
Merge pull request #1011 from w3c/submission/sideshowbarker/url-idl

Add URL IDL tests & drop stale (untested) URL IDLs