Previously, we had Webpack's ProvidePlugin injecting `node-fetch` as a
polyfill into Node builds, and web builds just relied on the `fetch`
built into the browser (if it's there).
Now, we use `cross-fetch` everywhere, and as a ponyfill instead of a
polyfill. This broke the unit tests in `fetch-tool.js` because they were
overriding `global.fetch` to mock `node-fetch`. Now, those tests use
`tap.mock` to mock `cross-fetch`. Of course, `tap.mock` didn't exist in
the old version of `node-tap`, so I also upgraded that.
