[fix][test] Fix invalid test NonPersistentTopicTest.testProducerRateLimit (apache#24957)

Signed-off-by: Dream95 <[email protected]>
(cherry picked from commit 0896c0a)
(cherry picked from commit 66b7f7c)

Author: Dream95

pulsar-broker/src/test/java/org/apache/pulsar/client/api/NonPersistentTopicTest.java

@@ -380,13 +380,14 @@ public void testProducerRateLimit() throws Exception {
             var producerName = BrokerTestUtil.newUniqueName("testProducerRateLimit");
             @Cleanup
             Producer<byte[]> producer = pulsarClient.newProducer().topic(topic).producerName(producerName).create();
-            byte[] msgData = "testData".getBytes();
             final int totalProduceMessages = 500;
             CountDownLatch latch = new CountDownLatch(totalProduceMessages);
             for (int i = 0; i < totalProduceMessages; i++) {
+                final int messageIndex = i;
                 executor.submit(() -> {
                     try {
-                        producer.send(msgData);
+                        String message = "my-message-" + messageIndex;
+                        producer.send(message.getBytes());
                     } catch (Exception e) {
                         log.error("Failed to send message", e);
                         failed.set(true);