test: refactoring test-cluster-worker-constructor

- Using assert.strictEqual instead assert.equal

PR-URL: https://github.com/nodejs/node/pull/9956


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Italo A. Casas <me@italoacasas.com>