Refactor tests to use nock or similar

[newhouse]

Experience working with this project has led me to the opinion that the testing suite could use some improvements. Particularly, the way we mock server responses is a bit contrived and confusing due to the way we're trying to deal with / simulate the node-fetch responses. We should just use nock or something similar to define the responses from a spoofed server, and let node-fetch do it's real thing.