Update tests to setup webdriver stuff in `jest-environment` and re-use one browser session instead of spawning one for each webdriver call to prevent creating too many BrowserStack sessions.
- [x] Move jest config from npm scripts to `jest.config.js`
- [x] Remove obsolete cross-env package (we don't need it anymore 🎉)
- [x] Fix bug where tests are not waiting for webdriver to be ready.
