Revert "cppgc: Rework testing GC infrastructure"

This reverts commit eb453679.

Reason for revert: Breaks MSAN: https://ci.chromium.org/p/v8/builders/ci/V8%20Linux%20-%20arm64%20-%20sim%20-%20MSAN/37053

Original change's description:
> 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}

Bug: chromium:1056170
Change-Id: Ieda44c07d08f837a6632f96b8db6d5bec87dd521
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2723216
Bot-Commit: Rubber Stamper <rubber-stamper@appspot.gserviceaccount.com>
Commit-Queue: Francis McCabe <fgm@chromium.org>
Cr-Commit-Position: refs/heads/master@{#73078}