[wasm][test] Clean up function body decoder unittest

This cleans up a few things after https://crrev.com/c/1458956.

R=titzer@chromium.org

Bug: v8:8562
Change-Id: I7904c195ee385f0aa8815447f440d8119601af3e
Reviewed-on: https://chromium-review.googlesource.com/c/1460467
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Reviewed-by: Michael Starzinger <mstarzinger@chromium.org>
Cr-Commit-Position: refs/heads/master@{#59567}