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 question, I believe that since the stale controlFlow is no longer being listened to by mocha, that any rejection that happens would be silently swallowed (since the mocha tests have already moved on to the next tests with new controlFlows tied to them). Question for you, it appears that `reset` might now be the thing that I am looking to call, since it appears to reset the controlFlow to its original state, but does not in itself cancel the currently executing promises? Is there a current function/functions that I should be calling instead? or is `reset` the correct thing to call?