This adds tests for the mock logic used in differential fuzzing. The
tests uncovered a couple of issues in the mock files that are also
fixed.

This also does some minor code clean up in the mock code.

Bug: chromium:1044942
Change-Id: I5b67f70f8b104bb681548f742ab863395a88360f
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2043843
Commit-Queue: Michael Achenbach <machenbach@chromium.org>
Reviewed-by: Mathias Bynens <mathias@chromium.org>
Cr-Commit-Position: refs/heads/master@{#66304}