我如何模拟express()来测试get是否被调用？

npm install express supertest --save-dev

const express = require('express');
const request = require('supertest');

const app = express();

app.get('/', (req, res) => {
  res.send('Hello World');
});

module.exports = app;

const request = require('supertest');
const app = require('./app');

describe('GET /', () => {
  it('responds with "Hello World"', (done) => {
    request(app)
      .get('/')
      .expect(200)
      .expect('Hello World', done);
  });
});

npm test