Tutorial 2, Step "Unit testing"

[rafaelmaroxa]

After following all the steps, and when it was time to build and run the tests on RootRouterTests.swift, the code wouldn't compile because LoggedInInteractableMock did not implement the gameDidStart method from OffGameListener (through LoggedInInteractable).

To fix it, I added the gameDidStart implementation with a counter for method calls, as well as a closure for providing an implementation stub.