[fix][test] fix flaky testNegativeAcksWithBackoff when batch enabled. (…

…#23986)

(cherry picked from commit eb7a4f3)

pulsar-broker/src/test/java/org/apache/pulsar/client/impl/NegativeAcksTest.java

@@ -256,9 +256,16 @@ public void testNegativeAcksWithBackoff(boolean batching, boolean usePartitions,
         long firstReceivedAt = System.currentTimeMillis();
         long expectedTotalRedeliveryDelay = 0;
         for (int i = 0; i < redeliverCount; i++) {
+            Message<String> msg = null;
             for (int j = 0; j < N; j++) {
-                Message<String> msg = consumer.receive();
+                msg = consumer.receive();
                 log.info("Received message {}", msg.getValue());
+                if (!batching) {
+                    consumer.negativeAcknowledge(msg);
+                }
+            }
+            if (batching) {
+                // for batching, we only need to nack one message in the batch to trigger redelivery
                 consumer.negativeAcknowledge(msg);
             }
             expectedTotalRedeliveryDelay += backoff.next(i);