GraphQL: Mock request support for testing

[shaps80]

@maticzav this is really a question for you to begin with.

I had trouble (as have others) running the tests since we don't seem to be able to get the local server running.

However I wonder why we even need to take that approach? I was thinking making use of URLProtocol would be far simpler and make it easier to run tests, directly from Xcode.

Can you comment on this, as perhaps I'm missing the point here ;)