test: expand test coverage of events.js

* test else path in emitMany function
* test calling removeAllListeners() in a event emitter instance
  with no events at all
* test calling removeListener() passing a event type that does
  not exist
* test calling eventNames() in a event emitter instance
  with no events at all

Refs: https://coverage.nodejs.org/coverage-ba776b3a56642d4c/root/events.js.html
PR-URL: https://github.com/nodejs/node/pull/10947


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Michael Dawson <michael_dawson@ca.ibm.com>
Reviewed-By: Gibson Fahnestock <gibfahn@gmail.com>