I'm not exactly fond of adding this method. If it's possible (even none-trivially) to inject an instance into a registry using [load](https://github.com/cakephp/cakephp/blob/3.0/Cake/Utility/ObjectRegistry.php#L64-L81) - I couldn't figure out how. 

It's possible to name a class to use, but that doesn't help as the mocked object needs to come from getMock()
It'd be possible using a mocked registry object mocking [_create](https://github.com/cakephp/cakephp/blob/3.0/Cake/ORM/BehaviorRegistry.php#L120-L130) but that then means simulating all of the logic implemented in _create _plus_ needing to account for any objects that are supposed to be handled normally.

The use case for that is adding a mocked behavior to the behaviorregistry to verify that the _right_ behavior+method is called, (you have probably already seen [the relevant diff](https://github.com/AD7six/cakephp/commit/a7bbe6864d7d9551b3cdde243229661ed180e65b)). For sane testing I think it's quite important to have the means to mock registered objects - is there a different way?