TSan complains in "isolates" tests otherwise. Also further reduce
virtual memory requirements of the sample test to address flaky
allocation failures on 32-bit platforms.

Change-Id: I26c9a59965009d7083876b4ff4836ee879d33350
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2000138
Commit-Queue: Jakob Kummerow <jkummerow@chromium.org>
Reviewed-by: Clemens Backes <clemensb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#65750}