test: refactor test-child-process-exec-error

* assert.equal() -> assert.strictEqual()
* regex -> .include()
* Change variable representing a function from `fun` to idiomatic `fn`
* var -> const

PR-URL: https://github.com/nodejs/node/pull/9780


Reviewed-By: Santiago Gimeno <santiago.gimeno@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Michael Dawson <michael_dawson@ca.ibm.com>
Reviewed-By: Evan Lucas <evanlucas@me.com>
Reviewed-By: Prince John Wesley <princejohnwesley@gmail.com>