Make sure that tests grow the new space in a safepoint. This fixes
races with concurrent allocation.

Bug: v8:10315
Change-Id: I6fce6740bc3c9385f18bbbcde4b06ba881a03635
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2428946Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Commit-Queue: Dominik Inführ <dinfuehr@chromium.org>
Cr-Commit-Position: refs/heads/master@{#70120}