Prevent search-specialpages.feature tests from running on enwiki

This should quiet failures that are expected (since code to satisfy
these tests is not currently on enwiki). Note, this should be re-enabled
for enwiki on 20 Feb 2014, when code to satisfy these tests should be
present on enwiki.

Change-Id: I43f45f46db491a8afd27b5baabd938b7ac999259