[8.x] Forget and clear a mocked/spied instance of an object in the container

[khalilst]

This PR was inspired by Jest's mockRestore() method.
It introduces new forgetMock() method which uses forgetInstance() method internally and has a better DX.

The use-case:
Consider we have multiple test methods using a mocked/spied instance of an object which is prepared in the setUp() method.
If in that class some tests want to run tests without mock, they can call the forgetMock() method.

[driesvints]

Can't we just also call this forgetInstance to keep it the same and generic?

[khalilst]

Just hard to guess without knowing what is going on behind the scene.