how to add new source file to an unit test target

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)

Start with Xcodeproj::Project::Object::FileReferencesFactory.new_reference and inspect how PBXGroup objects are created or accessed. Identify the relevant API documentation or example location, then document the steps for adding a source file to a unit test target and verify that the explanation covers the required group argument.

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