Just start learning Jest + @vue/test-utils. Here is a simple example for those people want to mock "vue-axios".
// @/components/Helloword.vue
<template>
  <div>
    <h1>Email: <span>{{ email }}</span></h1>
    <button @click="fetchData">Get Random Email</button>
  </div>
</template>
<script>
export default {
  name: 'HelloWorld',
  data() {
    return {
      email: '',
    };
  },
  methods: {
    async fetchData() {
      const res = (await this.axios.get('https://randomuser.me/api/')).data
        .results[0].email;
      this.email = res;
    },
  },
};
</script>
--
    // test/unit/example.spec.js
import { mount } from '@vue/test-utils';
import HelloWorld from '@/components/HelloWorld.vue';
import axios from 'axios';
jest.mock('axios', () => ({
  get: () =>
    Promise.resolve({
      data: {
        results: [{ email: 'mockAxios@email.com' }],
      },
    }),
}));
describe('HelloWorld.vue', () => {
  it('click and fetch data...', async (done) => {
    const wrapper = mount(HelloWorld, {
      mocks: {
        axios,
      },
    });
    await wrapper.find('button').trigger('click');
    wrapper.vm.$nextTick(() => {
      expect(wrapper.find('h1').text()).toContain('@');
      done();
    });
  });
});