Enable a minimal ESLint configuration and fix violations

[astorije]

More comprehensive version of #154

Configuration was created with npm run lint -- --init (plus minor tweaks like re-allowing console.log), and fixing violation was done with npm run lint -- --fix followed by very few minual fixes.
This will also be enforced by CI to make sure future PRs respect this.

This is how the config file was created:

The .eslintrc.yml file created is very minimal, I recommend going through https://eslint.org/docs/rules/ to see what is important to you after this PR.

⚠️ Note that some removals were the consequences of ESLint informing of a chain of unused variables. I tried my best but did not test this carefully beyond npm test so I might have removed test code that was supposed to stay.

[putermancer]

Due to another PR that I merged first, this one ended up having a minor merge conflict in tap_reporter.js. I checked out this branch locally, rebased your changes to resolve the TAP reporter conflict, and restored some of the original behavior that this PR had removed (probably because it was using a variable that was never meaningfully used when this PR was originally created).

Thanks for the PR, sorry for my slowness :)