Automatically test PhoneBlock on GitHub

[SchulteDev]

tl;dr: Build and test PhoneBlock on GitHub

This issue is somewhat related to #186, #202 and #204

In my opinion, it would be great if PhoneBlock would be automatically tested on GitHub.
In #186, there had been code changes which have broken PhoneBlock. Automatic tests could have highlighted this earlier. I can contribute PRs if you agree on this path @haumacher

From my point of view, the roadmap would be:

1. Being able to build PhoneBlock without tricks --> Build PhoneBlock immediately after checkout #204
   Optional: Introducing Maven Wrapper for having less system prerequisites when building PhoneBlock
2. Introducing a minimal "Test" GitHub action

Ongoing afterwards: Add tests to PhoneBlock.