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in there were out-of-date with the current URL spec, and regardless there's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.)