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.
Fixes#4686
Adds tests for @zeit/next-typescript so that we don't regress on this again.
I've fixed an issue in the `next` CLI too which caused lingering processes when the process gets force killed, which is what we do in the test suite, so it kept running if there was no manual quit.
