Skip unhandled promises for AsyncAwait performance test.

Bug: v8:1099632
Change-Id: I21d69d5700860f0b05fb8c6c90ea85dc28cb3890
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2274606Reviewed-by: Michael Stanton <mvstanton@chromium.org>
Commit-Queue: Camillo Bruni <cbruni@chromium.org>
Cr-Commit-Position: refs/heads/master@{#68631}