app.js

export default class App {
  constructor(userHandler){
     this.userHandler =  userHandler;
  }
  
 async testing(req, res) => {
  const { email } = req.body
  const r = await this.userHandler.findOne({email})
  res.json(r)
 }
}

// then in your tests
test('Pass POST v1/users/testing', async t => {
  sinon.stub(User, "findOne" () => { /* return an invalid user */ });
 
  const app = new App(User);
  const res = await request(app)
    .post('/api/v1/users/testing')
    .send({
      email: 'testing@email.com'
    })
  t.is(res.status, 400)
})