vuejs test emitted events with sinon spy

Foo.spec.js

import Vue from 'vue';
import Foo from '../src/components/Foo.vue';
import expect from 'expect';
import sinon from 'sinon';
import { mount } from '@vue/test-utils';

// Can comment out the mocks property and use this. Both work.
// Vue.prototype.$ebus = new Vue();

describe('Foo', () => {
  let wrp;

  beforeEach(() => {
    wrp = mount(Foo, {
      mocks: {
        $ebus: new Vue(),
      }
    });
  });

  it('displays the inserted message', () => {
    let input = wrp.find('input.msg');
    input.element.value = 'Message #1 from Foo';
    input.trigger('input');
    expect(wrp.find('.display-msg').html()).toContain('Message #1 from Foo');
  });

  it('sends an event through ebus', () => {

      let spy = sinon.spy(wrp.vm.$ebus, '$emit');

      let input = wrp.find('input.msg');
      input.element.value = 'Message #2 from Foo';
      input.trigger('input');

      console.log(spy.callCount);
      expect(spy.callCount).toBe(1);
      expect(spy.calledWithExactly('msg-sent', 'Message #2 from Foo')).toBe(true);

      // This just proves that I am listening to the event, not
      // that it is being emitted and listened on.
      //expect(wrp.vm.$ebus._events['msg-sen']).toBeFalsy();
      //expect(wrp.vm.$ebus._events['msg-sent']).toBeTruthy();
  });
});

Foo.vue

<template>
    <div>
        <input
            class="msg"
            type="text"
            v-model="msg"
            @input="sendMsg">

        <p class="display-msg" v-text="msg"></p>
    </div>
</template>

<script>

export default {
    data () {
        return {
            msg: ''
        };
    },

    created () {
        this.$ebus.$on('msg-sent', (data) => {
            //console.info('Foo.created evt msg-sent: ', data);
        })
    },

    methods: {
        sendMsg () {
           this.$ebus.$emit('msg-sent', this.msg);
        }
    }
}
</script>