Write a test for request cancellation on timeout

[sholladay]

When Ky times out the request with a TimeoutError, the expected behavior is for the request to be cancelled, which is accomplished using an AbortController. There was recently a regression that caused this not to happen. It is fixed by PR #238, however no test is included with that PR, as I was unable to figure out how to detect the cancellation on the server.

Investigate and implement a test for this behavior. I recommend doing so with a browser test in test/browser.js in order to test native fetch, as cancellation likely behaves a bit differently with node-fetch.