test: improve test-child-process-fork-and-spawn

* use const instead of var for required modules
* use assert.strictEqual instead of assert.equal
* remove unnecessary process.nextTick

PR-URL: https://github.com/nodejs/node/pull/10273


Reviewed-By: Santiago Gimeno <santiago.gimeno@gmail.com>
Reviewed-By: Evan Lucas <evanlucas@me.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Minwoo Jung <jmwsoft@gmail.com>
Reviewed-By: Italo A. Casas <me@italoacasas.com>