Issue #7563 - Allow mocha timeout to cancel flow.

This pull request is an attempt to address the issues described in https://code.google.com/p/selenium/issues/detail?id=7563

Good point (in regards to this not really being any different that normal timeout issues) I guess my only concern would be since the current promise (or subsiquent promises) could make actions with side-effects on the current web browser, it might impact other tests (that have already done their test setup code) but I'm not sure if that is something to worry about here.