hi @Georg and welcome to our community!
The latest version is 7.9 LTS. 
It has lots of new cool features.
The short answer is that we do not support the scanning of test files. We have this on the roadmap (see MMF-1451 - as you can see, it involves quite a lot of work). You can also add a vote on Allow enabling sonar rules for test sources and maybe add a comment in the thread to represent the dotnet community