cppgc: Rework testing GC infrastructure

Instead of moving the stand-alone logic to the base heap, allows
specific heaps to override their stand-alone GC behavior. This allows
CppHeap to reuse the unified heap bottlenecks and visitors for
testing. This works as long as any v8 references are empty as there is
no Isolate attached to the heap in this case.

- Reverts parts of https://crrev.com/c/2716291
- Relands parts of https://crrev.com/c/2718146

In addition, add tests covering v8::CppHeap and cppgc::Heap.

Bug: chromium:1056170
Change-Id: I47dc88c7f0e4961a1aadd60da9b05bff4dcfb27a
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2718612


Commit-Queue: Michael Lippautz <mlippautz@chromium.org>
Reviewed-by: Omer Katz <omerkatz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#73077}