Injecting test module with dagger2

I use Dagger2 in my android app. Basically I inject a HttpClient (interface) in MainActivity.

@Module
public class MainActivityModule{

   @Provides public HttpClient providesHttpComponent(){
        return new RealHttpClient();
    }
}

@Component( modules = MainActivityModule.class )
public interface MainActivityComponent {
   public MainActivity injectActivity(MainActivity);
}



public class MainActivity extends Activity {

   public void onCreate(Bundle saved){
      super.onCreate();

      Dagger_MainActivityComponent
        .builder()
        .mainActivityComponent(getModule())
        .build()
        .injectActivity(this);
   }


   protected MainActivityModule getModule(){
       return new MainActivityModule();
   }

}

So far so good, that works like expected. Now I want to write some unit tests for MainActivity where I want to use TestMainActivityModule instead of MainActivityModule.

@Module (overrides = true )
public class TestMainActivtiyModule extends MainActivityModule {

   @Provides public HttpClient(){
      return new MockHttpClient();
   }

}

My question is: How do I force MainActivity to use TestMainActivitiyModule instead of MainActivityModule? Is there a good solution for that?

My current approach is to use inheritance and to override getModule(), something like this

public class TestMainActivity extend MainActivity {

   @Override
   protected MainActivityModule getModule(){
      return new TestMainActivityModule();
   }
}

and to run unit test against TestMainActivity instead of MainActivity.

I guess it works, but one of the problems I'm facing with this approach is that I can't start TestMainActivity with an Intent because I can't specify it in AndroidManifest.xml

Does anyone know a better approach for unit testing with dagger2 on android?