This is cleanups that I forgot to include in the previous CLs or that
did not fit in any of them.

This is the eighth CL in a series to improve our module decoder tests
and make them more readable.

R=titzer@chromium.org

Bug: v8:8238
Change-Id: I0db04288f1efd9bb4642478d22c0edc8ac17e024
Reviewed-on: https://chromium-review.googlesource.com/c/1286669
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Reviewed-by: Ben Titzer <titzer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#56738}