pytest: a few basic tests

The tests can be executed by:
`pytest`

To see the output, use
`pytest --verbosity=4 --capture=tee-sys`

Server does not need to be running for the tests, pytest can start own django
service that responds (e.g., to REST API).
It is necessary to have created DB with `admin` user having a REST API token!
The `test_viewsets.py` sets DB in `django_db_setup()` fixture.