Also add a mixin for using the single-threaded default platform instead
of swapping it in with SetPlatformForTesting.

Bug: v8:12781
Change-Id: I304303e58ed713e5558d108cd7eb826c17abb40f
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3574553
Commit-Queue: Leszek Swirski <leszeks@chromium.org>
Auto-Submit: Leszek Swirski <leszeks@chromium.org>
Reviewed-by: Camillo Bruni <cbruni@chromium.org>
Commit-Queue: Camillo Bruni <cbruni@chromium.org>
Cr-Commit-Position: refs/heads/main@{#79829}