This harmonizes Swift integration tests with the rest of the repository,
to prepare for the internal integration test runner to run them. The
stripped down runner is kept compatible, so that current CI can still
use it now. Maybe it will be kept for developer use.
This PR includes:
* moving the integration tests inside `ql`
* editing `qlpack.yml` so that the internal runner can use it
* change database directory to be `test-db` rather than `db`
