unittests for hardware backends

I have tested in these commits the different unittests files for the ESS backend. Particularly, I have added the test test_connectors_serial.
Note that I have, I think, corrected small bugs in common/population.

I hope these are useful.
Rather than add the `include_only` argument, just put these tests in `test_hardware_brainscales.py` rather than in `scenarios`.