Configure testwikidata to use the "special" site link group

Change-Id: I5bf5c77162480497dafabd16941e7a2b51d2e4ba
Add client settings needed for the other projects beta feature

No-op with the currently deployed Wikibase version, but
will get relevant once I07abcbafbd1 goes out.

Change-Id: Id3d93f44953b34b317318e0b319a42462129e95b