test: add test for repl.defineCommand()

It also tests displayPrompt by checking for '> '.

PR-URL: https://github.com/nodejs/node/pull/3908


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Sakthipriyan Vairamani <thechargingvolcano@gmail.com>
Reviewed-By: Jeremiah Senkpiel <fishrock123@rocketmail.com>