This implements parsing part of customizable <select> spec update.
See whatwg/html PR #10548.
Two failing subtests in `html5lib_innerHTML_tests_innerHTML_1.html`
and `customizable-select/select-parsing.html` are due to the spec
still disallowing `<input>` inside `<select>`, even though Chrome
has already implemented this behavoir (see whatwg/html#11288).
Since we don't support the "variant" meta tag stuff in WPT, I've simply
copied the test files here, and then test.js looks at the filename to
figure out which test function to use.
This incrases our coverage of the HTML parser substantially by also
invoking it via document.write() one-shot, and character-at-a-time.
