Compute the length of more fields automatically, in particular names.

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

R=titzer@chromium.org

Bug: v8:8238
Change-Id: I1bd27f45380d82af2d7319f15ac7e37d5b9e4081
Reviewed-on: https://chromium-review.googlesource.com/c/1283077
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Reviewed-by: Ben Titzer <titzer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#56721}