Bug 1387447 - Update accessibility pointer-events expectations. r=yzen

With a WebDriver-conforming Element Click implementation, the element
click intercepted error is returned when an element with pointer-events:
"none" causes the click to hit the underlying element.

This patch does not functionally change anything yet about the
accessibility tests, but splits disabled_accessibility_elementIDs into
two lists, aria_disabled_elements and pointer_events_none_elements, in
anticipation of moving Marionette to use a different click implementation.

In the future, the ARIA tests will fail with "element not accessible"
errors as they do now, but the pointer-events tests will fail with
"element click intercepted" instead.

MozReview-Commit-ID: Ks1hyUVyLK7

--HG--
extra : rebase_source : 20dbcc228955626cd2d1617aa055f29f4e5928a4