[TASK] Stop checking tests/ with Psalm

[oliverklee]

This will pave the way towards moving Psalm to Phive
until we find a better solution.

As the tests call the production code, Psalm would mark the called
code as unused. So we need to disable some checks for unused code.

Fixes #879

[oliverklee]

This needs more changes for the Psalm configuration.

[JakeQZ]

Looks good. And, thinking about it, trying to check the testing code was actually holding us back checking the actual code. In the long run we can reintroduce it, but I'd rather see an almost empty baseline from src/ first.