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

@@ -110,9 +110,9 @@ void testBasic() {
     void testExpirations() throws InterruptedException {
         eventHandle2.release(true);
         Thread.sleep(TEST_TIMEOUT.multipliedBy(2).toMillis());
-        assertThat(acknowledgementSetManager.getAcknowledgementSetMonitor().getSize(), equalTo(0));
         await().atMost(TEST_TIMEOUT.multipliedBy(3))
                 .untilAsserted(() -> {
+                    assertThat(acknowledgementSetManager.getAcknowledgementSetMonitor().getSize(), equalTo(0));
                     assertThat(result, equalTo(null));
                 });
     }