Dependency Injection while Unit Testing
I have few thoughts about this situation: 


Controllers should not contain business logic, services should. Therefor unit testing a controller is not a good idea in the first place.
If you are unit testing your controllers "getResource" method, why do mock the controller itself? You should mock the HttpClient. (that's the strong part of DI)
First you mock the classes which your constructor needs as arguments. Then you just create your object, which will you test like this: $example_controller = new ExampleController($http_client_mock); And that's it.