This increases readability of the wasm-stepping test significantly.
Drive-by: Use more 'let' instead of 'var'.

R=yangguo@chromium.org

Change-Id: If80ba3a4b92cd3ab1c994e17fb8f40f5526517da
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1789298Reviewed-by: Yang Guo <yangguo@chromium.org>
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#63616}