KAFKA-19180 Fix the hanging testPendingTaskSize (#19526)

The check for `scheduler.pendingTaskSize()` may fail if the thread pool
is too slow to consume the runnable objects

Reviewers: Ken Huang <[email protected]>, PoAn Yang
 <[email protected]>, Chia-Ping Tsai <[email protected]>

Author: Yunyung

Diff for: core/src/test/scala/unit/kafka/utils/SchedulerTest.scala

@@ -220,10 +220,14 @@ class SchedulerTest {
     scheduler.scheduleOnce("task1", task1, 0)
     scheduler.scheduleOnce("task2", () => latch2.countDown(), 5)
     scheduler.scheduleOnce("task3", () => latch2.countDown(), 5)
-    assertEquals(2, scheduler.pendingTaskSize())
+    retry(30000) {
+      assertEquals(2, scheduler.pendingTaskSize())
+    }
     latch1.countDown()
     latch2.await()
-    assertEquals(0, scheduler.pendingTaskSize())
+    retry(30000) {
+      assertEquals(0, scheduler.pendingTaskSize())
+    }
     scheduler.shutdown()
     assertEquals(0, scheduler.pendingTaskSize())
   }