Support several expect/assert cycles for same mock

[tcard]

You should be able to do something like this:

mock := &SomeInterfaceMock{...}
subject := NewRealCode(mock)

assertMock := mock.Describe().(...)
subject.DoSomethingUsingMock()
assertMock(t)

assertMock = mock.Describe().(...)
subject.DoAnotherThingUsingMock()
assertMock(t)

Right now Describe() is a method on a mocker, which is a builder for mock objects, and ultimately produces the mock object alongside the assertMock function. It should act on an already existing mock object instead.