If we don't have slack and we want to add an element to the
TransitionArray, we would create a new TransitionArray. The background
hread, however, can be holding a pointer to the old transitions. This
test tests that this is safe to do, i.e the background thread reading
the old TransitionArray.

To make sure that we are testing that, we can add more synchronization
via an extra semaphore.

Bug: v8:7790
Change-Id: Ie454d79282ac267d3527269e8490baced979aa45
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2323351Reviewed-by: Nico Hartmann <nicohartmann@chromium.org>
Commit-Queue: Santiago Aboy Solanes <solanes@chromium.org>
Cr-Commit-Position: refs/heads/master@{#69126}