MINOR: Fix LogDirFailureTest flake

Ensure that `TestUtils.waitUntilTrue(..)` is blocked on both send completed and a new leader being assigned

Author: Gardner Vickers <gardner@vickers.me>

Reviewers: Dhruvil Shah <dhruvil@confluent.io>, Dong Lin <lindong28@gmail.com>

Closes #5695 from gardnervickers/log-dir-failure-test-fix

core/src/test/scala/unit/kafka/server/LogDirFailureTest.scala

@@ -178,13 +178,15 @@ class LogDirFailureTest extends IntegrationTestHarness {
         }
     }
 
-    // Wait for producer to update metadata for the partition
     TestUtils.waitUntilTrue(() => {
       // ProduceResponse may contain KafkaStorageException and trigger metadata update
       producer.send(record)
       producer.partitionsFor(topic).asScala.find(_.partition() == 0).get.leader().id() != leaderServerId
     }, "Expected new leader for the partition", 6000L)
 
+    // Block on send to ensure that new leader accepts a message.
+    producer.send(record).get(6000L, TimeUnit.MILLISECONDS)
+
     // Consumer should receive some messages
     TestUtils.waitUntilTrue(() => {
       consumer.poll(0).count() > 0