classes with Test in their name

[abeceda4]

Hi,
 
there, in the code, is lot of classes with "Test" in their name. For example 
KafkaProducerTest, KafkaAdminClientTest or ClusterConnectionStateTest etc. 
Could you tell me please, if these classe are used somewhere in Kafka by 
regular way or
if these classes are used only for testing purposes?
 
Thank you very much
 
Mrkvica