Mock server responses for core API tests

[rfk]

The tests currently talk to a live server, which is great for dev testing but not ideal for an automated suite. We should change them to use mocked responses by default, but still be able to talk to a live server if requested explicitly.

[vladikoff]

We have these mocks for the fxa-js-client that might be useful: https://github.com/mozilla/fxa-js-client/tree/master/tests/mocks

[leplatrem]

It might result quite complex to write a single piece of code that would fit with both mocked and real server responses.

But I guess having two test suits could make sense! One with mocked server responses focused on unit tests, and another one talking to a real server focused on functional tests.

(my two cents)

[Natim]

I started using mocks for #23 and #28