test: refactor /parallel/test-cluster-uncaught-exception.js to ES6

Replaces function expressions with ES6 arrow functions as well as
improve the comparison operator to check if operands are of same types.

PR-URL: https://github.com/nodejs/node/pull/9239


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Rich Trott <rtrott@gmail.com>