Adds testing API that can only be used after enabling it on a heap.
The call that enables testing is only provided via v8_for_testing or
cppgc_for_testing build targets which protects against misusing from
production code.

Change-Id: I24a8f5543a2bb479481384e2c555d231383e5d12
Bug: chromium:1056170
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2667513Reviewed-by: Hannes Payer <hpayer@chromium.org>
Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Reviewed-by: Omer Katz <omerkatz@chromium.org>
Commit-Queue: Michael Lippautz <mlippautz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#72503}