Retrofit 2.0 Android Unit Testing using Local JSON

I'm new to Retrofit 2.0 and I want to ask about the best way to do unit test using this, especially for asynchronous request.

I found a good article about it here and I'm interested on using the local JSON static files for my unit testing since it will be faster in my opinion and doesn't always require internet connection, but it won't work when I implement it on Retrofit 2.0. Is it possible to do this in Retrofit 2.0?

Or maybe can somebody here help me with good references or maybe some good examples about how to do these unit tests?

Sorry for my bad English.