[tests] Run pytest tests in runtests.py

[nemesifier]

@atb00ker can we change the runtests.py script to make it run tests with pytest?
If it finds the SAMPLE_APP env var it could automatically select the right ini file.
Or we may also avoid the ini files completely and just call the raw command from the script.

This would simplify things. I often forget about running those tests.

[atb00ker]

When I was trying this last time, there was some problem, but I don't remember the details, I'll check again and ping back! 😄

[atb00ker]

possible example: openwisp/openwisp-notifications#57 (comment)