Setup unit tests.

Nobody has claimed this yet.

- [ ] Convert the installation scripts into tested functions that are executed according to some order.
Ideally, first setup the self-hosted GitLab CI in this repo: https://github.com/Simple-Setup/Self-host-GitLab-Server-and-Runner-CI then make that Style Guide Compliant, and after that start converting the files/scripts in this repository into unit-tested functions.

The preference for first fixing the GitLab CI in that repo, is such that pull requests on this repo then will get automatic CI, which is a convenient way of working.
The preference for first making that repo Google Style Guide compliant is subjective, I think it is wise to take a step back in creating new functionalities, and to clean up the repositories, right now. I believe this allows a higher rate of creating new functionalities in the long run, improves code quality, and makes it easier for others to contribute to this repository. Another important factor is that it is a step towards applying the TruCol protocol to this repository, (which requires a high test coverage and proper test setup).

No files, tests, or entry points are named. Start by reviewing the installation scripts in this repository and the referenced Self-host-GitLab-Server-and-Runner-CI repository, then establish the required execution order, CI setup, style compliance, and unit-test coverage as separate decisions. Done means the scripts are tested functions executed in order and pull requests receive automatic CI.

Written by the indexing model from the issue text.