Thanks Matthias, I'm interested in your workshop and documentation.
Where are you going to give them?
In the meantime I've done the workshop, so here is a first little help how to include a GeoGebra file into a Moodle test (provided you have installed the plugin).
In your test choose GeoGebra question type and than either link to a file on GeoGebra materials or drag a file into the appropriate section.
As the test activity always checks for a true/false (boolean) variable, the simplest GeoGebra test file just consists of such a variable (see attached file).
What you have to do now, is to tell GeoGebra which variable to use, e.g. boolean (the name of the variable in the GeoGebra file).
That's all.