[tests] Test serialization of redis-cluster connection with @medium.

Serializing an instance of a redis-cluster connection with its nodes
can take more than 1 seconds on extremely slow hardware.

See issue #220 for reference.

tests/Predis/Connection/Aggregate/RedisClusterTest.php

@@ -731,6 +731,7 @@ class RedisClusterTest extends PredisTestCase
     }
 
     /**
+     * @medium
      * @group disconnected
      */
     public function testCanBeSerialized()