Write tests for aria-labelledby and inclusion of supplying elements in accessibility tree

[extra808]

Elements that are the source an adjacent element's name should remain in the reading order unless explicitly excluded.

Example, both elements should be read as "Test A" by assistive technologies:

<a id="test-a" href="#">Test A</a> <button aria-labelledby="test-a"></button>

Related WebKit (on iOS) bug: 285048