test: refactor the code in test-child-process-spawn-loop.js

* use const and let instead of var
* use assert.strictEqual instead of assert.equal
* use arrow functions

PR-URL: https://github.com/nodejs/node/pull/10605


Reviewed-By: Benjamin Gruenbaum <benjamingr@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>