Fix flaky DefaultAcknowledgementSetManagerTests.testExpirations

Move the monitor size assertion into the await().untilAsserted() block.
The assertion was placed after Thread.sleep() but before await(),
assuming the cleanup thread had already run. Under CI load, the cleanup
may not have completed in time, causing intermittent failures.

Signed-off-by: Sotaro Hikita <bering1814@gmail.com>

Author: lawofcycles

data-prepper-core/src/test/java/org/opensearch/dataprepper/core/acknowledgements/DefaultAcknowledgementSetManagerTests.java

@@ -38,7 +38,7 @@
 @ExtendWith(MockitoExtension.class)
 class DefaultAcknowledgementSetManagerTests {
     private static final Duration TEST_TIMEOUT = Duration.ofMillis(400);
-    private static final Duration EXPIRY_TEST_TIMEOUT = Duration.ofMillis(100);
+    private static final Duration EXPIRY_TEST_TIMEOUT = Duration.ofMillis(400);
     private DefaultAcknowledgementSetManager acknowledgementSetManager;
     private ScheduledExecutorService callbackExecutor;
 
@@ -109,10 +109,10 @@ void testBasic() {
     @Test
     void testExpirations() throws InterruptedException {
         eventHandle2.release(true);
-        Thread.sleep(TEST_TIMEOUT.multipliedBy(2).toMillis());
-        assertThat(acknowledgementSetManager.getAcknowledgementSetMonitor().getSize(), equalTo(0));
+        Thread.sleep(20);
         await().atMost(TEST_TIMEOUT.multipliedBy(3))
                 .untilAsserted(() -> {
+                    assertThat(acknowledgementSetManager.getAcknowledgementSetMonitor().getSize(), equalTo(0));
                     assertThat(result, equalTo(null));
                 });
     }