fix MaxListenersExceededWarning

[kobenguyent]

Workers
✔ should run simple worker (548ms)
✔ should create worker by function (481ms)
✔ should run worker with custom config (539ms)
(node:21160) MaxListenersExceededWarning: Possible EventEmitter memory leak detected. 51 suite.before listeners added to [EventEmitter]. MaxListeners is 50. Use emitter.setMaxListeners() to increase limit
(node:21160) MaxListenersExceededWarning: Possible EventEmitter memory leak detected. 11 beforeExit listeners added to [process]. MaxListeners is 10. Use emitter.setMaxListeners() to increase limit
✔ should able to add tests to each worker (399ms)
✔ should able to add tests to using createGroupsOfTests (397ms)
(node:21160) MaxListenersExceededWarning: Possible EventEmitter memory leak detected. 51 test.passed listeners added to [EventEmitter]. MaxListeners is 50. Use emitter.setMaxListeners() to increase limit
(node:21160) MaxListenersExceededWarning: Possible EventEmitter memory leak detected. 51 test.failed listeners added to [EventEmitter]. MaxListeners is 50. Use emitter.setMaxListeners() to increase limit