cluster: test events emit on cluster.worker

v0.10 and node docs specific that in a worker, the 'message' and 'error'
event emits on process, and on cluster.worker.

Reviewed-By: Fedor Indutny <fedor@indutny.com>
Revert "repl: fix overwrite for this._prompt"

This reverts commit 7166b55015261de8ab69758320f3d9159b3eaadd.