Improve test coverage

[Stannislav]

At the moment the unit test coverage is about 48%, and large parts of the code are untested.

[FrancescoCasalegno]

This Issue is not measurable, hence there's no Definition of Done and it is not actionable as of now.

We should make it more precise, e.g.:

• what is the target coverage percentage?
• which parts of the code do we want to cover?