test: replace function with arrow function

1. Among the list of Code and Learn,
I solved the unfinished task of replacing function with arrow function:
https://github.com/nodejs/code-and-learn/issues/72#issuecomment-345667395

2. Replace arrow function with shorter property syntax
Arrow function makes `this` lexical scope.
But toString expects evaluate `this` in runtime.

3. Replace this with null
makeBlock does not need `this`.
update `this` with `null` to clarify the intent.

PR-URL: https://github.com/nodejs/node/pull/17345


Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Jon Moss <me@jonathanmoss.me>
Reviewed-By: Yosuke Furukawa <yosuke.furukawa@gmail.com>