DOM: Add more Observable iterable tests

[chromium-wpt-export-bot]

This CL adds and supplements a few tests:

1. First we modify the existing "subscribe with aborted signal" tests.
   Specifically, we expand their assertions to not only assert that
   next() isn't ever called, but make more assertions about the
   iterator protocol getter and function invocations in general.
2. Second, we modify the test that asserts next() is not called
   when you subscribe with an unaborted signal, but that signal gets
   aborted while the iterator protocol methods are called during
   subscription of the Observable. We expand the assertions in the
   same way as (1), and combine the two separate tests into one that
   covers both sync and async iterators, also to match (1).
3. Finally, this CL adds a sync iterable version of the test added in
   https://crrev.com/c/6199630. The test scenario is: you subscribe to
   a sync iterable with an unaborted signal that gets aborted while
   obtaining the iterator (just like (2)), BUT while getting the
   iterator, an error is thrown. The tests asserts that the error is
   reported to the global before we consult the aborted signal and
   stop the subscription process. This ensures that the exception is
   not swallowed, but is appropriately surfaced, even though the
   subscription is aborted.

This corresponds with the spec PR:
WICG/observable#192.

R=masonf

Bug: 363015168
Change-Id: Ida605c49a2d73cd407a9dc3c392d6b2f338855be
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6202182
Commit-Queue: Dominic Farolino <[email protected]>
Reviewed-by: Mason Freed <[email protected]>
Cr-Commit-Position: refs/heads/main@{#1412315}

[wpt-pr-bot]

The review process for this patch is being conducted in the Chromium project.