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>

test/parallel/test-cluster-worker-constructor.js

@@ -3,9 +3,9 @@
 // validates correct behavior of the cluster.Worker constructor
 
 require('../common');
-var assert = require('assert');
-var cluster = require('cluster');
-var worker;
+const assert = require('assert');
+const cluster = require('cluster');
+let worker;
 
 worker = new cluster.Worker();
 assert.strictEqual(worker.suicide, undefined);