A Retrofit Client that reads JSON files from disk and returns them as the response (to be handled by GSON). Make sure the context you pass is from the instrumentation app and put your JSON files in `res/raw` in the instrumentation project. Inspired by Ruby's `vcr` gem.

A `GET` request to `/articles` will look for the JSON file `res/raw/get_articles.json`, a `POST` request to `/articles/123/` will look for the JSON file `res/raw/post_articles_123.json`, etc.

We use `setScenario()` if we want to test alternate responses so that a failed login `POST` to `/login` will read from `res/raw/failed_login_post_login.json` (and the happy path can use `post_login.json`).