Split logic for running the tests and displaying the results

[SebastianZ]

At the moment the logic for executing the tests and outputting their results is strongly entangled.

This hinders further improvement regarding both and also makes it harder to read the code.

E.g. we want to execute the tests only once but update the display depending on the chosen filter (see #212 (comment)).

Therefore the code for running the tests and displaying the test results should be split and run separately from each other.

Sebastian