Функция-заглушка не выполняется при выполнении теста с использованием sinon и mocha

Я пытаюсь смоделировать функцию, чтобы пройти тесты, используя sinon и mocha.

приложение.js:

const express = require('express');
const { isValid } = require('./utils/index');

const config = require('./config.json')[process.env.NODE_ENV || 'development']


const app = express();

app.get('/', (req, res)=> {


    const url = config.url;
   try {
    const validUrl = isValid(url)
    .then(() => {
        return res.redirect(`https://${url}`);
    })
    .catch(() => {
        return res.status(400).send('Unable to redirect to the given url');
    })
   } catch(error) {
      return res.send('Internal Server Error') 
   }




})

const port = process.env.port || 3000;

const server = app.listen(port, ()=> {
    console.info('server listens on 127.0.0.1:3000');
})

module.exports = {server, app};

конфиг.json:

{
"development": {
    "url": "www.stackoverflow.com"
}, 
"test": {
    "url": "www.stackoverflow.com"
}

}

утилиты/index.js:

const http = require('http');


module.exports.isValid = (url) => {
    const options = {
        method: 'HEAD',
        host: url
    }
const promise = new Promise((resolve, reject) => {
    const req = http.request(options, () => {
        return resolve(true)
    })
    req.on('error', () => {
        return reject(new Error('Not valid'))
    })

    req.end();
})

return promise;
}

тест/index.js:

const request = require('supertest');
const chai = require('chai');
const sinon = require('sinon');
const index = require('../utils/index')
const { expect } = chai;

const { server } = require('../app');
const {url} = require('../config.json')['test'];



describe('isValid Test', () => {

    it('Should redirects an error when the url is not valid', async() => {

        const stub = sinon.stub(index, 'isValid');
        stub.withArgs(url).returns(Promise.reject(new Error('Not Valid')));
        const { status } = await request(server).get('/');

        expect(status).to.equal(400);

    })
})

Когда я выполняю тест, я получаю эту ошибку:

    (node:23622) UnhandledPromiseRejectionWarning: Error: Not Valid
    at Context.it (/home/hs/perso/mockTests/chaiMock/test/index.js:17:52)
    at callFn (/home/hs/perso/mockTests/chaiMock/node_modules/mocha/lib/runnable.js:387:21)
    at Test.Runnable.run (/home/hs/perso/mockTests/chaiMock/node_modules/mocha/lib/runnable.js:379:7)
    at Runner.runTest (/home/hs/perso/mockTests/chaiMock/node_modules/mocha/lib/runner.js:535:10)
    at /home/hs/perso/mockTests/chaiMock/node_modules/mocha/lib/runner.js:653:12
    at next (/home/hs/perso/mockTests/chaiMock/node_modules/mocha/lib/runner.js:447:14)
    at /home/hs/perso/mockTests/chaiMock/node_modules/mocha/lib/runner.js:457:7
    at next (/home/hs/perso/mockTests/chaiMock/node_modules/mocha/lib/runner.js:362:14)
    at Immediate.<anonymous> (/home/hs/perso/mockTests/chaiMock/node_modules/mocha/lib/runner.js:425:5)
    at runCallback (timers.js:705:18)
    at tryOnImmediate (timers.js:676:5)
    at processImmediate (timers.js:658:5)
(node:23622) UnhandledPromiseRejectionWarning: Unhandled promise rejection. This error originated either by throwing inside of an async function without a catch block, or by rejecting a promise which was not handled with .catch(). (rejection id: 1)
(node:23622) [DEP0018] DeprecationWarning: Unhandled promise rejections are deprecated. In the future, promise rejections that are not handled will terminate the Node.js process with a non-zero exit code.
url:  www.stackoverflow.com
    1) Should redirects an error when the url is not valid


  0 passing (122ms)
  1 failing

  1) isValid Test
       Should redirects an error when the url is not valid:

      AssertionError: expected 302 to equal 400
      + expected - actual

      -302
      +400

      at Context.it (test/index.js:22:27)
      at process._tickCallback (internal/process/next_tick.js:68:7)