Creating a fetch in the page, and then asynchronously aborting it, should be reflected in fetchEvent.request.signal.

Something like (in page):

const controller = new AbortController();
const f = fetch('foo', { signal: controller.signal });
setTimeout(() => controller.abort(), 1000);

In service worker:

self.onfetch = fetchEvent => {
  fetchEvent.request.signal.onabort = () => console.log('aborted');
  ...
}

'aborted' should be logged after a second.

First thing to do is to write some tests.