how to add new source file to an unit test target

[xxhank]

I tried Xcodeproj::Project::Object::FileReferencesFactory.new_reference, but this method needs a group(PBXGroup) argument, and I cannot find how to get a group(PBXGroup)

[Desnoo]

An group is simply said a grouped set of files represented as an virtual folder inside of xcode.
I think the root group is the project itself.