benchmark: Fix crash in net benchmarks

Net benchmarks create partial event emitters that do not have all of the
required event emitter functions. They currently mock out `on`, `once`,
and `emit` functions. This change mocks out `prependListener` as well to
avoid crashing in `_stream_readable`.

PR-URL: https://www.github.com/nodejs/node/pull/6407
Fixes: https://www.github.com/nodejs/node/issues/6405


Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Minwoo Jung <jmwsoft@gmail.com>
Reviewed-By: Matthew Loring <mattloring@google.com>
Reviewed-By: James M Snell <jasnell@gmail.com>