Make the Features class mockable so it's easier to test

[caendesilva]

Right now some tests get a lot of extra complexity when trying to fulfil certain requirements that could just as well be mocked.

Could be something for intermediary participators of Hacktoberfest?

Thinking an interface like Features::mock('method', 'return value') could be nice. Of course both the mocking feature itself needs to have tests, bonus points if you refactor some existing test code to use the mocking feature to demonstrate it "in action".