Testing: add unit (module) test facilities

[AshFungor]

Hey, @riebl!

I had a look into OMNeT++ test utility and I feel that it's a bit limited and feels "packed": adding source code alongside configuration files all in different format and style seems a bit brutal to me. I thought that we may build up a small number of facilities for unit (OMNeT++ modules) tests, which would be orchestrated and validated by pytest. The idea is launch small networks of (ideally) a couple of modules and then check if it works ok.

What do you think?

[riebl]

I think building "small networks" for checking modules is the way to go. Instead of putting effort into any orchestration tooling in the first place, I would rather like to see how such networks could look like, which test input data is required and how it is provided, and how checks (test assertions) are applied.

An alternative to pytest might be CTest, i.e. adding test configurations to our CMake configuration.