Best practices to make functional test in MVP+RxJava

I am moving my app to MVP and to use functional testing. The first application Activity is a Splash Activity, which call a service requesting some geo-positioned data which then will be showed in a map. Being I'm using MVP, now the service connection is made in an Interactor class. And being I am using RxJava+Retrofit the data is retrieved on a Subscriber's callback method, which then put the data in a Singleton to cache it. I am not interested in unit test. I want to test it functionally, with a real service call. So, please, any clue will be very welcomed. Thanks in advance.