test: refactor test-child-process-kill

* var -> const
* assert.equal() -> assert.strictEqual()

PR-URL: https://github.com/nodejs/node/pull/9903


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Franziska Hinkelmann <franziska.hinkelmann@gmail.com>