KAFKA-19384 The passing of BrokerRegistrationRequestTest is a false positive (#20338)

Fixes a false positive in `BrokerRegistrationRequestTest` caused by
`isMigratingZkBroker`, and migrates the test from Scala to Java.

Reviewers: Chia-Ping Tsai <[email protected]>

Author: apalan60

checkstyle/suppressions.xml

@@ -49,6 +49,7 @@
     <suppress checks="MethodLength|JavaNCSS|NPath" files="DescribeTopicPartitionsRequestHandlerTest.java"/>
     <suppress checks="CyclomaticComplexity" files="ListConsumerGroupTest.java"/>
     <suppress checks="ClassFanOutComplexity|CyclomaticComplexity|MethodLength|ParameterNumber|JavaNCSS|ImportControl" files="RequestConvertToJson.java"/>
+    <suppress checks="ImportControl" files="BrokerRegistrationRequestTest.java"/>
 
     <!-- Clients -->
     <suppress id="dontUseSystemExit"