[wasm][test] Fix memory64 test

There are fives bytes total which are expected to be different from
zero. We were only handling one of them when checking random positions
in the array. This was leading to random failures.

R=manoskouk@chromium.org

Bug: v8:11621
Change-Id: Iac231d8b35fcbfbbc837c8e9134401cb8a2519ac
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2810783


Reviewed-by: Manos Koukoutos <manoskouk@chromium.org>
Commit-Queue: Clemens Backes <clemensb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#73860}