Tests: Add more logging to SearchPreferenceTests so that we can know which preference is failing.

core/src/test/java/org/elasticsearch/search/preference/SearchPreferenceTests.java

@@ -49,6 +49,7 @@ public class SearchPreferenceTests extends ElasticsearchIntegrationTest {
         client().admin().cluster().prepareHealth().setWaitForStatus(ClusterHealthStatus.RED).execute().actionGet();
         String[] preferences = new String[] {"_primary", "_local", "_primary_first", "_prefer_node:somenode", "_prefer_node:server2"};
         for (String pref : preferences) {
+            logger.info("--> Testing out preference={}", pref);
             SearchResponse searchResponse = client().prepareSearch().setSize(0).setPreference(pref).execute().actionGet();
             assertThat(RestStatus.OK, equalTo(searchResponse.status()));
             assertThat(pref, searchResponse.getFailedShards(), greaterThanOrEqualTo(0));