test: refactor assert.equal, update syntax to ES6

PR-URL: https://github.com/nodejs/node/pull/10190


Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Sakthipriyan Vairamani <thechargingvolcano@gmail.com>
Reviewed-By: Gibson Fahnestock <gibfahn@gmail.com>