App.test.tsx

import { loginUser } from "./functions";
import { mocked } from "ts-jest/utils";

jest.mock("./functions");
const loginUserMock = mocked(loginUser);

describe("App", () => {
  beforeEach(() => {
    loginUserMock.mockResolvedValue({
      loginSucceeded: false,
    });
  });

  // ...
});