This CL adds a linear search test in a DescriptorArray in a known flat
object in the background thread, while the main thread exercises the
same DescriptorArray.

Also sets the foundation for the follow-ups tests in background threads.

Bug: v8:7790
Change-Id: I0e99508204808baaf605161d2eeb717eabe712fb
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2207147
Commit-Queue: Santiago Aboy Solanes <solanes@chromium.org>
Reviewed-by: Georg Neis <neis@chromium.org>
Reviewed-by: Dominik Inführ <dinfuehr@chromium.org>
Reviewed-by: Michael Stanton <mvstanton@chromium.org>
Cr-Commit-Position: refs/heads/master@{#68299}