A testing method was missing a code ref scope, making fuzzers fail.

R=mstarzinger@chromium.org

Bug: chromium:952759
Change-Id: Ib9d485fad85f66ca358a769a4e52777f68367991
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1571605Reviewed-by: Michael Starzinger <mstarzinger@chromium.org>
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#60909}