Adding a 'PrepareFunctionForOptimization' call will prevent the test
case in question to become flaky when stress testing bytecode flushing.

R=jarin@chromium.org

Bug: v8:9123
Change-Id: If192ebf571d3cd4f0d1ee31bc3f6313b74d3c866
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1564202
Auto-Submit: Simon Zünd <szuend@chromium.org>
Commit-Queue: Sigurd Schneider <sigurds@chromium.org>
Reviewed-by: Sigurd Schneider <sigurds@chromium.org>
Cr-Commit-Position: refs/heads/master@{#60803}