This uses the current dev build by default but can be overidden with
CODE_SERVER_TEST_ENTRY (for example to test a release or some other
version).
Each instance has a separate state directory. This should make
parallelization work.
This also means you are no longer required to specify the password and
address yourself (or the extension directory once we add a test
extension). `yarn test:e2e` should just work as-is.
Lastly, it means the tests are no longer subject to yarn watch randomly
restarting.
