Add config to allow test.wikidata to be a Wikibase client

This patch is best with I1cd5ab0 in WikimediaMessages also
deployed, though this can be deployed without and can wait
until next regular deploy of WikimediaMessages.

Change-Id: Ia4433b2fc918bc7b883f2b25ba2d293c4ba5ae80
Merge "Add config to allow test.wikidata to be a Wikibase client"