[wasm][test] Compute section length automatically

Instead of specifying the byte length of a section manually, just
compute it automatically from the bytes given. Manual computation is
particularly difficult because of the macros involved, which can expand
to several bytes.
This is not a pure refactoring, it also fixes several occasions where
we calculated the length wrong.

Drive-by: Add some ENTRY_COUNT macro uses.

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

R=titzer@chromium.org

Bug: v8:8238
Change-Id: I0d2ceb751fc8e5625ffdf4189d4b5253aecc2541
Reviewed-on: https://chromium-review.googlesource.com/c/1283075
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Reviewed-by: Ben Titzer <titzer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#56718}