We might have to wait for background tasks to complete (e.g. when
awaiting ax  WebAssembly.instantiate) and not just other promises.

Bug: v8:12568
Change-Id: Ibf99074d7a3c924b77b7959006087a05d39ffd3b
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3423777Reviewed-by: Andreas Haas <ahaas@chromium.org>
Commit-Queue: Camillo Bruni <cbruni@chromium.org>
Cr-Commit-Position: refs/heads/main@{#78871}