This fixes a general race with stack-scoped `TestPlatform` which
may go out of scope while tasks on workers are still running.

Add a barrier for workers, implemented through tasks, to synchronize
destruction of `TestPlatform`.

While this fixes general races, such short-lived platforms still
break if tasks cache the global platform pointer.

Bug: v8:12635
Change-Id: Ifc6ecc29f0e2b7297ca52051eae9bd81013b60ce
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3536651Reviewed-by: Leszek Swirski <leszeks@chromium.org>
Commit-Queue: Michael Lippautz <mlippautz@chromium.org>
Cr-Commit-Position: refs/heads/main@{#79587}