TESTS: Fix IndexStatsIT#testFilterCacheStats (#36143)

* Test randomly failed because of background merges
   * Fixed by force merging down to a single segment
* Closes #32506

server/src/test/java/org/elasticsearch/indices/stats/IndexStatsIT.java

@@ -23,6 +23,7 @@
 import org.elasticsearch.action.DocWriteResponse;
 import org.elasticsearch.action.admin.cluster.node.stats.NodesStatsResponse;
 import org.elasticsearch.action.admin.indices.create.CreateIndexRequest;
+import org.elasticsearch.action.admin.indices.forcemerge.ForceMergeResponse;
 import org.elasticsearch.action.admin.indices.stats.CommonStats;
 import org.elasticsearch.action.admin.indices.stats.CommonStatsFlags;
 import org.elasticsearch.action.admin.indices.stats.CommonStatsFlags.Flag;
@@ -1006,7 +1007,6 @@ private void assertCumulativeQueryCacheStats(IndicesStatsResponse response) {
         assertEquals(total, shardTotal);
     }
 
-    @AwaitsFix(bugUrl = "https://github.com/elastic/elasticsearch/issues/32506")
     public void testFilterCacheStats() throws Exception {
         Settings settings = Settings.builder().put(indexSettings()).put("number_of_replicas", 0).build();
         assertAcked(prepareCreate("index").setSettings(settings).get());
@@ -1056,7 +1056,11 @@ public void testFilterCacheStats() throws Exception {
             persistGlobalCheckpoint("index");
             flush("index");
         }
+        ForceMergeResponse forceMergeResponse =
+            client().admin().indices().prepareForceMerge("index").setFlush(true).setMaxNumSegments(1).get();
+        assertAllSuccessful(forceMergeResponse);
         refresh();
+
         response = client().admin().indices().prepareStats("index").setQueryCache(true).get();
         assertCumulativeQueryCacheStats(response);
         assertThat(response.getTotal().queryCache.getHitCount(), greaterThan(0L));