For some reason the compiler was optimizing away the reference to the
object in WeakContainerTest.ConservativeGCTracesWeakContainer and thus
not finding it conservatively.
This CL revises the tests such that the compiler is no longer able to
optimize references away.

Bug: v8:12824
Change-Id: Ie598a1cf1124c2983a6c61fd4e990734d36f5832
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3610627Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Commit-Queue: Omer Katz <omerkatz@chromium.org>
Reviewed-by: Anton Bikineev <bikineev@chromium.org>
Cr-Commit-Position: refs/heads/main@{#80257}