Test: Add code coverage for backend

[drikusroor]

This PR adds

• The coverage package, which allows us to generate code coverage reports.
• The genbadge package, which allows us to generate code coverage badges.
• A script to the workflow that runs the coverage command to give us an inline code coverage report.
• A script to the workflow that runs the coverage command (only for pushes to develop and main) with the xml option. Thish allows us to generate a badge. The badge then gets pushed to a dedicated code-coverage-badges branch on Github and this badge image is then used to be displayed in the README.md on top. See also here