test: add tests for eslint rules

This adds tests for the custom eslint rules in this repository, using
the `RuleTester` test utility bundled with eslint.

PR-URL: https://github.com/nodejs/node/pull/16138


Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Yuta Hiroto <hello@about-hiroppy.com>
Reviewed-By: James M Snell <jasnell@gmail.com>