Write tests for implicit value calculation for ARIA positioning attributes, once testing is unblocked

[smhigley]

Related to #2104

Track writing tests for the new browser requirements for implicitly calculating values for aria-posinset, aria-setsize, and aria-level, based on the new browser calculation requirements introduced in the ARIA PR (formerly some requirements were in Core AAM, but they were more ambiguously defined).

Suggested tests include:

• tree + treeitem -- all three attributes
• tree + treeitem, with an aria-owns relationship -- all three attributes
• nested comments -- all three attributes
• list + listitem
• menu + menuitem
• tablist + tab
• listbox + option
• radiogroup + radio
• table + row

@cookiecrook mentioned the tests being currently blocked by testability on the ARIA WG call.

[cookiecrook]

The scope of this issue is pretty large... Most of this and more will be unblocked (and is planned to be tested) once AOM #203 or another technique makes it possible, but I'm not certain the value of keeping this particular issue open. I think it's covered by #90 and other issues. Okay to close this one @smhigley?