Add unit tests with Jest

[jdnichollsc]

Remember to export the jest setup file...

[radko93]

If anyone has a problem with setting jest right now create a file __mocks__/react-native-inappbrowser-reborn.js and then add there:

export default {
  open: jest.fn(),
  close: jest.fn(),
  openAuth: jest.fn(),
  closeAuth: jest.fn(),
  isAvailable: jest.fn(),
}

Expand when you need more for testing.