Fix mediawiki-core-regression-master

The Interwiki class caches lookups, so ApiQueryTest.php's use of "en:"
was failing for mediawiki-core-regression-master because something else
had already made a call for that prefix. The easy fix is to use a
different prefix that is very unlikely to ever be used in a different
test.

Change-Id: I43ca554d5b5465d760eae03d1f76b8c3bf589210