Provide an artificial way to toggle circuit breakers for testability

[dhh1128]

Do we need more than the simple directTransition()? What we'd like to do is make it possible to exercise degraded behavior without having to mock every condition that triggers them normally.

A concern I have with this feature is that it could be co-opted by nefarious entities to alter program flow in an attack/vulnerability situation. So we need to consider security implications.