"flutter create" can generate a basic driver test

"flutter create" adds option `--with-driver-test` that adds
dependencies to `flutter_driver` in `pubspec.yaml` and creates
a basic driver test runnable via `flutter drive
--target=test_driver/e2e.dart`

"flutter drive" new options:

- `--keep-app-running` tells the driver to not stop the app after tests
  are done
- `--use-existing-app` tells the driver to not start a new app but use
  an already running instance