Fetch: Add tests for AbortSignal's abort reason

[nidhijaju]

This PR adds test cases to check the functionality of AbortSignal's abort reason when aborting fetch.

See https://github.com/whatwg/fetch/pull/1343 for accompanying spec changes.

[yutakahirano]

The added cases look good. I think we should test that the service worker can observe the fetch abort and see the contents of the abort reason.

[nidhijaju]

The added cases look good. I think we should test that the service worker can observe the fetch abort and see the contents of the abort reason.

I added a new test for this case, so hopefully it's looking better :) PTAL. Thank you!

[noamr]

I'm missing a test that shows that the serialization happens on abort. I would expect something like a live JS object that increments a counter every time it's accessed. The reason observed by the SW should show a "frozen" version from when the abort took place while other callers would still get the live object.

[nidhijaju]

I'm missing a test that shows that the serialization happens on abort. I would expect something like a live JS object that increments a counter every time it's accessed. The reason observed by the SW should show a "frozen" version from when the abort took place while other callers would still get the live object.

Apologies for the delay! I added a test, but I'm not sure if it's exactly what you were looking for. Would you be able to take a look? Thank you!