DOM: Simplify Observable async iterable subscription

This CL slightly simplifies the Observable async iterable subscription
flow by moving the abort signal check *after* the null `ScriptIterator`
check. This has two effects:
  1. Allows us to remove one call to `ClearAbortAlgorithm()`, since we
     are assigning the abort algorithm handle *after* the null
     `ScriptIterator` check, and that check is the last opportunity to
     cancel the subscription
  2. Brings the async iterable subscription path inline with the *sync*
     subscription path, which is ordered similarly [1]

This has a very subtle behavior change, so this CL also adds a test for
this case. The spec will be updated accordingly.

[1]:
https://source.chromium.org/chromium/chromium/src/+/main:third_party/blink/renderer/core/dom/observable.cc;l=1990-2019;drc=4b00956a8d971cf5ec8ec45a105b5dd8802d4a42

R=masonf

Bug: 363015168
Change-Id: Idf43bed901b1339d3581ce97c1cd02a0ac6b8b48
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6199630
Reviewed-by: Mason Freed <masonf@chromium.org>
Commit-Queue: Dominic Farolino <dom@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1411976}