test: improve test-child-process-exec-buffer

* use const instead of var for required modules
* use assert.strictEqual instead of assert.equal
* use assert.strictEqual instead of assert.ok

PR-URL: https://github.com/nodejs/node/pull/10275


Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Italo A. Casas <me@italoacasas.com>