test2.wikipedia.org (created in 2011) is a test wiki inside the production cluster (like test.wikipedia.org). test2 is primarily used to trial and debug global and cross-wiki features in conjunction with test.wikipedia.org and test.wikidata.org.
As of January 2016 test.wikipedia.org and test2.wikipedia.org both get the "next" wmf-branch in the first phase.
test2 was originally created to ensure testability of all Het deploy states throughout the deployment phases (where test.wikipedia.org would be the first wiki to get the next version and test2.wikipedia.org the last – together with the Wikipedias).
test.wikipedia and test2.wikipedia are both served from the production cluster behind the main load balancers serving live scapped code from the apache servers.