test: refactor test-child-process-pass-fd

Add a comment explaining the test (especailly why it forks 80 processes.
Use destructuring and an arrow function callback.

PR-URL: https://github.com/nodejs/node/pull/17596


Reviewed-By: Khaidi Chu <i@2333.moe>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Minwoo Jung <minwoo@nodesource.com>