Improve the test suite

This MR implements a number of changes to our current test suite:

• Re-implement unit tests for communication routines.
• Implement new generic unit tests for 'NetworkServer'.
• Enable running test_regression.py with missing model frameworks.
• Add a --cpu-only mode for unit tests.
• Ensure multiprocessed functional tests run on CPU (to avoid a race on hardware resource).

On the side, it fixes 'WebsocketServer.stop' backend code.