innerText getter has many whitespace-related subtleties

[pshaughn]

html/dom/elements/the-innertext-idl-attribute/getter.html tests innerText in many situations. One failure involves <ruby> and I don't understand where that one's coming from at all, but most of them involve getting back different whitespace than what's expected.

https://html.spec.whatwg.org/multipage/dom.html#inner-text-collection-steps describes an elaborate series of steps that are mostly about figuring out where linebreaks go; I think servo's implementation (mostly in layout/query.rs) is out of sync with the spec.

Layout 2020 seems to have a lot of the relevant functionality stubbed out completely right now.