:use_route in controller specs inside engines

If you have controller specs inside your Rails engine, you'll need to pass use_route: :my_engine when doing the request. Otherwise you'll get an UrlGenerationError since the dummy app is not running and the engine is not mounted!

describe MyEngine::MainController, type: :controller do
  it "does something" do
    get :index, params: { use_route: :my_engine }
    # ...