Replace tests with pytest runner

[roaffix]

• Remove custom test runner tool
• Add pytest runner (there is no need to install it locally from requirements because it is used as setup test requirement) These tests are not checking the correctness of the executed method and they are nothing more than smoke tests. So, they are put in the appropriate tests subfolder.

The tests can be run with the python setup.py test command.

Minimal coverage was set to fit the current coverage value -- 70% (was 72.3%, actual 71.86% after removing some prints)