Introduce runs again

Yeah, I know, we've been through this before. However, I changed my mind again, now that I stopped rushing and gave myself some time to think 😄

This adds the idea of "runs" back again. A "run" is just an application of the whole test database to a specific execution of tests and is the data that templates would consume. It should make it easier for us to implement multihost support and all the other features. This also lets us finally get rid of template sorting in kpet-db (see corresponding MR).