Enhancement | Improve tests coverage

[Franr]

Describe the Feature

Based on coverage and coveralls.io, we should improve our test coverage to avoid bugs when refactoring code.

Expected Behavior

The idea is to:

• improve coverage over files that show a big lack of it and are frequently used (moslty modules/ files)
• establish a coverage threshold (80%) and enforce developers to stick to it (with the introduction of a GitHub Action)