Как правильно проверить ценность файла cookie при использовании React-cookie?

Вы высмеяли useCookies, чтобы иметь реализацию jest.fn(). Но баннер cookie использует его, вероятно, так:

const [cookies, setCookie, removeCookie] = useCookies(['cookie-name']);

//jest.fn returns undefined so the previous line is equivalent to:

const [cookies, setCookie, removeCookie] = undefined;

//And then it tries to get the first, second, and the third indexed item for undefined which leads to your error "undefined is not iterable" and it is true.

Правильное решение — высмеять react-cookie, но у него должен быть тот же контракт.

import { fireEvent, render, screen } from '@testing-library/react'
import { useCookies } from '@testing-library/react'

//we mock it initially for all
jest.mock('react-cookie', () => {
    __esModule: true,
    useCookies: jest.fn(() => [
       // mock for cookies
       [],
       //mock for setCookie
       jest.fn(),
       //mock for removeCookie
       jest.fn()
    ])
})

const mockedUseCookies == useCookies as jest.Mock<any>

const getCookiesSpies = () => {

  const setCookieMock = jest.fn();
  const removeCookieMock = jest.fn();

  mockedUseCookies.mockImplementation(() => ([
       // mock for cookies
       [],
       //mock for setCookie
       setCookieMock,
       //mock for removeCookie
       removeCookieMock 
    ])
  ]));

  return {setCookieMock, removeCookieMock }
}

describe('Cookies Banner', () => {

   beforeEach(() => {
       // THIS IS IMPORTANT SO THAT EACH TEST IS IN ISOLATION
      jest.clearAllMocks();
   });

  it('Cookie will get proper values after click on Accept Cookies button', () => 
  {
       // OVERIDE setCookies BEFORE RENDERING !!!!

       const {setCookieMock, removeCookieMock } = getCookiesSpies();
        render(
            <CookiesBanner {...someCookieData}></CookiesBanner>
        )

        const acceptCookiesBtn = screen.getByTestId('acceptCookiesBtn')
        fireEvent.click(acceptCookiesBtn)

        //How do I check if the cookies have the right value here???
        expect(setCookieMock).toHaveBeenCalledWith('expected-cookie-name', 'expected-cookie-value')
    })
  })

Это также можно расширить, чтобы вводить результат в параметр useCookies - cookie при вызове его для каждого теста.