test: fix test-cluster-send-handle-large-payload

On macOS, the parent process might not receive a message if it
is sent to soon, and then subsequent messages are also sometimes not
received.

(Is this a bug or expected operating system behavior like the
way a file watcher is returned before it's actually watching the file
system on/ macOS?)

Send a second message after a delay on macOS.

While at it, minor refactoring to the test:

* Blank line after loading `common` module per test-writing guide
* Wrap arrow function in braces where implicit return is not needed
* Remove unnecessary unref in subprocess

PR-URL: https://github.com/nodejs/node/pull/14780
Fixes: https://github.com/nodejs/node/issues/14747


Reviewed-By: Anna Henningsen <anna@addaleax.net>