Fix unit test (apache#6006)

### Motivation

Since apache#5599 merged, it introduce some conflict code with master branch, maybe the reason is apache#5599 not rebase with master

### Verifying this change

This is a test change

(cherry picked from commit 275854e)

Author: codelipenghui

pulsar-broker/src/test/java/org/apache/pulsar/broker/service/BrokerServiceTest.java

@@ -946,9 +946,9 @@ public void testStuckTopicUnloading() throws Exception {
                 .get(mlFactory);
         assertNotNull(ledgers.get(topicMlName));
 
-        org.apache.pulsar.broker.service.Producer prod = spy(topic.producers.values().get(0));
+        org.apache.pulsar.broker.service.Producer prod = (org.apache.pulsar.broker.service.Producer) spy(topic.producers.values().toArray()[0]);
         topic.producers.clear();
-        topic.producers.add(prod);
+        topic.producers.put(prod.getProducerName(), prod);
         CompletableFuture<Void> waitFuture = new CompletableFuture<Void>();
         doReturn(waitFuture).when(prod).disconnect();
         Set<NamespaceBundle> bundles = pulsar.getNamespaceService().getOwnedServiceUnits();