test: improve test-cluster-worker-constructor.js

* use let and const instead of var
* use assert.strictEqual instead assert.equal

PR-URL: https://github.com/nodejs/node/pull/10396


Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Italo A. Casas <me@italoacasas.com>
Reviewed-By: Santiago Gimeno <santiago.gimeno@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>