Test was asserting heap size before and after GC. With background
thread allocation those assertions might not hold.

Bug: v8:10315
Change-Id: I4f8c0f6d0b80040b3c89f85e801416abb29ed30e
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2421999Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Commit-Queue: Dominik Inführ <dinfuehr@chromium.org>
Cr-Commit-Position: refs/heads/master@{#70034}