hasRegisteredListener(eventName, listenerName) {
        return !!(this.listeners(eventName).find(listener => (listener.name === listenerName)));
    }
This is method is semi-hack to make sure listeners are only registered once
during the lifetime of the process. And example problem it solves is
registering duplicate listeners between Ghost instance reboots when running tests.
@param {String} eventName
@param {String} listenerName named function name registered as a listener for the event
@returns {Boolean}