[PATCH v2] dts: Testbed And Node Configuration Split

[Luca Vizzarro]

This is a good change, but I'd like to review it once you adapt it to 
the Pydantic-based config, as it may look quite different.

While we are splitting things I suggest to create two new source files 
under `config` for each sections:

   conf/
     __init__.py # where Configurations stays and brings everything together
     test_run.py # to hold the `test_run` and all the related models
     node.py     # as above but for `node`

This will hopefully make it easier to read.

Best,
Luca