Improve mocks and render/tests/test-attributes

[pygy]

This is a preliminary PR before I move on with the actual fixes for #1804 and #1595 redux.

The tests commented out are failing, but shouldn't.

[dead-claudia]

LGTM.

I do wonder why we're using elem.attributes instead of the more idiomatic, shorter elem.getAttribute(). It'd simplify that part of the mock by a lot.

[pygy]

Thanks for the review @isiahmeadows

I agree, using getAttribute() would enable simpler mocks, but what we have works... I'm not in a hurry to replace them. I changed .nodeValue to .value because attributes soon won't be implementing the Node interface anymore. FIrefox already gives deprecation warnings.

[dead-claudia]

I agree with that, and it's probably for the best. I might some day address that, but for now, meh.

¯\(ツ)/¯