refactor: extract root-level tests directory

When reading through the code, I got confused a few times while trying to figure out what certain targets or files were doing, only to eventually realise that they were only needed by tests.

This change introduces the tests root-level directory (in line with the guidelines for repository structure). I've decided to mimic the folder structure of the labgrid subfolder being tested (e.g. tests for labgrid/my-folder go under tests/my-folder).

This creates a bit of separation between test and code, which may be undesired. I'm very much open to feedback on this.