Test expectations for aria-hidden and focused elements

[cookiecrook]

Once #101 and/or #125 unblock this testing, add tests to verify the focusability testing expectations of what happens to aria-hidden once a hidden-but-focusable item gets keyboard focus.

WPT side blocked by

• Investigate keyboard operability testing #101
• Write tests for keyboard utils investigation #125

ARIA spec side blocked by

• Expectations for aria-hidden and focused elements w3c/aria#2181