Fix for Vuex mocking not working

gistfile1.txt

https://github.com/vuejs/vue-test-utils/issues/116#issuecomment-521366881

```
-- MyComponent.spec.js --

import { shallowMount } from '@vue/test-utils'
import store from '@/store'
import router from '@/router'
import MyComponent from '@/components/MyComponent'

describe('MyComponent', () => {
  const wrapper = shallowMount(MyComponent, {
    router,
    store,
  })
  it('render without crashing', () => {
     ...
  })
})
```

```
 -- store.js --

import Vue from 'vue'
import Vuex from 'vuex'

const test = {
  namespaced: true,
  state: { ... },
  getters: { ... },
  actions: { ... },
  mutations: { ... },
}

Vue.use(Vuex)

const debug = process.env.NODE_ENV !== 'production'

export default new Vuex.Store({
  modules: {
   test,
  },
  strict: debug,
});

```

```
-- router.js --

import Vue from 'vue'
import VueRouter from 'vue-router'

import MyComponent from '@/components/MyComponent'

const routes = [
  { path: '/', name: 'My Component', component: MyComponent },
]

Vue.use(VueRouter)

export default new VueRouter({
  mode: 'history',
  routes,
})
```