Tests: Add information about shards that are not closed when the filter cache gets closed.

src/main/java/org/elasticsearch/indices/cache/filter/IndicesFilterCache.java

@@ -266,7 +266,7 @@ public class IndicesFilterCache extends AbstractComponent implements QueryCache,
     @Override
     public void close() {
         assert shardKeyMap.size() == 0 : shardKeyMap.size();
-        assert shardStats.isEmpty();
+        assert shardStats.isEmpty() : shardStats.keySet();
         assert stats2.isEmpty() : stats2;
         cache.clear();
     }