Run the Promises/A+ test suite in real browsers

We're currently running the Promises/A+ tests only in Node and only on master. That mean compat is mostly untested and on master we don't know if it really works in all the browsers.

We should get the test suite run in real browsers. IE8 will be a problem since the suite uses getters heavily but even getting it all run in other browsers would be a huge win.

It's not a must-have for the beta IMO but we should have it ready before 3.0.0 final so I'll set the milestone.

Refs gh-1722 Refs gh-1996