MINOR: Make MockClient#poll() more thread-safe (#5942)

It used to preallocate an array of responses and then complete each response from the original collection sequentially. The problem was that the original collection could have been modified (another thread completing the response) while this was hapenning

clients/src/test/java/org/apache/kafka/clients/MockClient.java

@@ -276,7 +276,6 @@ public class MockClient implements KafkaClient {
         maybeAwaitWakeup();
         checkTimeoutOfPendingRequests(now);
 
-        List<ClientResponse> copy = new ArrayList<>(this.responses);
         // We skip metadata updates if all nodes are currently blacked out
         if (metadataUpdater.isUpdateNeeded() && leastLoadedNode(now) != null) {
             MetadataUpdate metadataUpdate = metadataUpdates.poll();
@@ -287,9 +286,11 @@ public class MockClient implements KafkaClient {
             }
         }
 
+        List<ClientResponse> copy = new ArrayList<>();
         ClientResponse response;
         while ((response = this.responses.poll()) != null) {
             response.onComplete();
+            copy.add(response);
         }
 
         return copy;

clients/src/test/java/org/apache/kafka/clients/admin/AdminClientUnitTestEnv.java

@@ -35,6 +35,10 @@ import java.util.Map;
  * To use in a test, create an instance and prepare its {@link #kafkaClient() MockClient} with the expected responses
  * for the {@link AdminClient}. Then, use the {@link #adminClient() AdminClient} in the test, which will then use the MockClient
  * and receive the responses you provided.
+ *
+ * Since {@link #kafkaClient() MockClient} is not thread-safe,
+ * users should be wary of calling its methods after the {@link #adminClient() AdminClient} is instantiated.
+ *
  * <p>
  * When finished, be sure to {@link #close() close} the environment object.
  */