test: improve assertions in child-process-execsync

Improve assertions in test-child-process-execsync by removing unneeded
third arguments and replacing equal checks with assert.ok() where
appropriate.

PR-URL: https://github.com/nodejs/node/pull/22016


Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Jon Moss <me@jonathanmoss.me>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>