EventBus.js

import Vue from 'vue'
const EvtBus = new Vue()

EvtBus.addEvent = (evt, fn) => {
  EvtBus.$on(evt, () => {
    fn()
  })
}

export { EvtBus }

EventBus.test.js

import { EvtBus } from './EventBus'

describe('EventBus', () => {
  it('adds a new event using addEvent', () => {
    const evtToAdd = () => {}
    EvtBus.addEvent('test', evtToAdd)
    expect(EvtBus._events['test']).toBeTruthy()
  })
})

Test.test.js

import { shallow } from 'vue-test-utils'
import Test from './Test'

describe('Test', () => {
  it('renders', () => {
    const wrapper = shallow(Test)
    wrapper.find('button').trigger('click')
    expect(wrapper.emitted().test.length).toBe(1)
  })
})

Test.vue

<template>
  <div>
    <button @click="go">Emit event</button>
  </div>
</template>

<script>
  import { EvtBus } from './EventBus'

  export default {
    name: 'Test',

    created () {
      EvtBus.addEvent('test', () => {
        console.log('Add this function to event bus')
      })
    },

    methods: {
      go () {
        this.$emit('test')
        EvtBus.$emit('test')
      }
    }
  }
</script>

<style scoped>
</style>