Wikidata Query Service and test.wikidata.org both use the same favion:
- https://www.wikidata.org/static/favicon/testwikidata.ico for query service
- https://test.wikidata.org/static/favicon/testwikidata.ico for Test.Wikidata
The file is however apparently the same. Since this continuously confused me in my browser tabs, I suggest to use different favicons. I do not have a preference which one to keep and which one to change.