test: use eslint to fix var->const/let

Manually fix issues that eslint --fix couldn't do automatically.

PR-URL: https://github.com/nodejs/node/pull/10685


Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Roman Reiss <me@silverwind.io>