userTest.js

import { expect, should } from 'chai';
const url = '../index';

const testObject = {
  emptyEmail: {
    email: '',
    password: 'tuij'
  },
  wrongEmailFormat: {
    email: 'danielopeyemi*',
    password: 'opeyemi'
  },
  correctInput: {
    email: '[email protected]',
    password: 'opeyemi'
  }
};

descibe('API Endpoint to Login Users on medium.com', () => {
  it('Should return error if email field is empty', () => {
    chai.request(url)
      .post('/auth/signin')
      .send(testObject.emptyEmail)
      .then((res) => {
        expect(res).to.have.status(400);
        res.body.should.have.property('message').eql('Email field is empty');
    });
  });
  
   it('Should return error if email syntax is wrong', () => {
    chai.request(url)
      .post('/auth/signin')
      .send(testObject.wrongEmailFormat)
      .then((res) => {
        expect(res).to.have.status(400);
        res.body.should.have.property('message').eql('Email format is wrong');
    });
  });
  
   it('Should return ok if email and password field is correct', () => {
    chai.request(url)
      .post('/auth/signin')
      .send(testObject.correctInput)
      .then((res) => {
        expect(res).to.have.status(200);
        res.body.should.have.property('message').eql('Login Successful');
    });
  });
})

Looks good to me.