Currently the system test reads the new leader from a broker log statement on completion of become-leader state transition. The log statements can change with time and can also move around.
We can read the leader info from zookeeper given a topic, partition. The path is #2 at https://cwiki.apache.org/confluence/display/KAFKA/Kafka+data+structures+in+Zookeeper. The Zookeeper data structures are unlikely to change in foreseeable future.