Write GUI integration tests

[e-caste]

After the #104 is done both CLI and GUI-side, we will need to write some tests to verify that the pytarallo integration works correctly, by importing and calling the functions that talk to the TARALLO.

[e-caste]

For the GUI: we can use http://github.com/pytest-dev/pytest-qt

For the CLI:

• write stdin:https://stackoverflow.com/questions/38723140/i-want-to-use-stdin-in-a-pytest-test
• read stdout: https://docs.pytest.org/en/stable/capture.html
• some other examples from another of our repositories: https://github.com/WEEE-Open/falce/blob/master/falce