WMF_testsystem is one of the MediaWiki instances in the Wikiation Extension Testing Environment. This system is intended to mimick the English Wikipedia.
The way this configuration is build is distinctly different from the English Wikipedia. This system is scratch installed, it does not share the files needed for CentralAuth and MergeAccount, the Cite and the SpecialCite are in separate folders and last but not least, there are a few extensions that I have not figured out how to install yet.
What I really liked was that it took me only five minutes to rebuild my environment once Wikipedia was updated. Three minutes to change my installation script and two minutes to run it.