Refactor nps tests

[kentcdodds]

Let's get rid of sinon and just use jest's built-in mock functions.

Also, we use jest.mock in a pretty sub-optimal way. We really shouldn't be doing jest.mock calls inside of functions. Those should go at the top of the module where we're calling it.

Also, doing inline jest.mock is not the best. We should probably write these mocks in __mocks__ directories (see the Jest docs).

[Miklet]

#162 removed sinon from tests. Mocks still need improvement 🙂