This PR upgrades `jest-worker` and `jest-cli` to the latest pre-release version, also removed `jest-circus` which is included in Jest by default. `jest-worker@next` includes a fix for memory leak that we need (https://github.com/facebook/jest/pull/11187).
Fixes#22925. This will also improve the OOM issue for `next dev` #15855.
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.
