KAFKA-13312; 'NetworkDegradeTest#test_rate' should wait until iperf server is listening (#11344)
KAFKA-13312; 'NetworkDegradeTest#test_rate' should wait until iperf server is listening (#11344)

Reviewers: Jason Gustafson <68c46a606457643eab92053c1c05574abb26f861@confluent.io>
TiVo/kafka,guozhangwang/kafka,guozhangwang/kafka,lindong28/kafka,TiVo/kafka,lindong28/kafka,apache/kafka,apache/kafka,TiVo/kafka,lindong28/kafka,apache/kafka,guozhangwang/kafka,apache/kafka,TiVo/kafka,guozhangwang/kafka,lindong28/kafka