cppgc: Implement testing APIs

These are used by v8_wrapper/heap_test_utilities.* in Blink.
See crrev.com/c/2787126 for usage.

Bug: chromium:1056170
Change-Id: I329b1823f2ac21181a3536577ed72bee3d591347
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2786842


Commit-Queue: Omer Katz <omerkatz@chromium.org>
Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#73805}