ATTA: Consider third result value for known issues

[joanmarie]

Having sat down to resume debugging my ATTA this evening, I noticed there are a number of failures, all of which turned out to be the same bug (Gecko isn't exposing STATE_CHECKABLE on my platform).

I'm thinking it might be nice to add a third result value, so there's PASS, FAIL, and KNOWN_ISSUE (or something to that effect). This value would not be used by test authors because I don't think anything different should be done in how the tests are written or run. However, I can envision test runners and others examining the current state of implementations looking into a failure that someone else already knows about.

This should be easy to add to the ATTAs as an ATTA expectation. I'll come up with something to demonstrate it hopefully tomorrow (aka today).