[beta] controller unit-testability: stubbable `controllers` property

Wouldn't it be nice to be able to unit test controllers
without necessitating knowledge of the container?
Well? Now you can! Today is your lucky day.

Removed the `readOnly`-ness of the `controllers` CP so that it could be
stubbed out for unit tests. I suspect the worries of
epic self trolls are overblown particular when it comes
at the expense of unit-testability without a container.