[Bugfix] Fix flakiness in TieredSpilloverCacheStatsIT.testClosingShard() (#18204)

* Fix flakiness in testClosingShard()

Signed-off-by: Peter Alfonsi <petealft@amazon.com>

* Update modules/cache-common/src/internalClusterTest/java/org/opensearch/cache/common/tier/TieredSpilloverCacheStatsIT.java

Co-authored-by: Andrew Ross <andrross@amazon.com>
Signed-off-by: Peter Alfonsi <peter.alfonsi@gmail.com>

---------

Signed-off-by: Peter Alfonsi <petealft@amazon.com>
Signed-off-by: Peter Alfonsi <peter.alfonsi@gmail.com>
Co-authored-by: Peter Alfonsi <petealft@amazon.com>
Co-authored-by: Andrew Ross <andrross@amazon.com>

Author: 3 people

modules/cache-common/src/internalClusterTest/java/org/opensearch/cache/common/tier/TieredSpilloverCacheStatsIT.java

@@ -430,7 +430,7 @@ public void testClosingShard() throws Exception {
 
         // Closing the index should close the shard
         assertAcked(client().admin().indices().delete(new DeleteIndexRequest("index")).get());
-        assertEquals(new ImmutableCacheStats(0, 0, 0, 0, 0), getTotalStats(client));
+        assertBusy(() -> assertEquals(new ImmutableCacheStats(0, 0, 0, 0, 0), getTotalStats(client)));
     }
 
     private void startIndex(Client client, String indexName) throws InterruptedException {