[hotfix] Fix FlinkKafkaConsumerBaseTest.testClosePartitionDiscovererWithCancellation

Make a copy of AbstractPartitionDiscoverer#getAllTopics before modifying it.

Author: AHeise

flink-connector-kafka/src/main/java/org/apache/flink/streaming/connectors/kafka/internals/AbstractPartitionDiscoverer.java

@@ -19,6 +19,7 @@
 
 import org.apache.flink.annotation.Internal;
 
+import java.util.ArrayList;
 import java.util.HashSet;
 import java.util.Iterator;
 import java.util.List;
@@ -132,7 +133,7 @@ public List<KafkaTopicPartition> discoverPartitions() throws WakeupException, Cl
                     newDiscoveredPartitions =
                             getAllPartitionsForTopics(topicsDescriptor.getFixedTopics());
                 } else {
-                    List<String> matchedTopics = getAllTopics();
+                    List<String> matchedTopics = new ArrayList<>(getAllTopics());
 
                     // retain topics that match the pattern
                     Iterator<String> iter = matchedTopics.iterator();