Harden FrozenEngineTests#testCircuitBreakerAccounting

This test tries to compare the CB stats from an InternalEngine
and a FrozenEngine but is subject to segement merges that might finish
and get committed after we read the breaker stats. This can cause
occational test failures.

Closes #36207

Author: s1monw

x-pack/plugin/core/src/test/java/org/elasticsearch/index/engine/FrozenEngineTests.java

@@ -7,6 +7,7 @@
 
 import org.apache.lucene.index.DirectoryReader;
 import org.apache.lucene.index.FilterDirectoryReader;
+import org.apache.lucene.index.NoMergePolicy;
 import org.apache.lucene.search.MatchAllDocsQuery;
 import org.apache.lucene.search.ReferenceManager;
 import org.apache.lucene.search.TopDocs;
@@ -138,16 +139,17 @@ public void testCircuitBreakerAccounting() throws IOException {
         final AtomicLong globalCheckpoint = new AtomicLong(SequenceNumbers.NO_OPS_PERFORMED);
         try (Store store = createStore()) {
             CountingRefreshListener listener = new CountingRefreshListener();
-            EngineConfig config = config(defaultSettings, store, createTempDir(), newMergePolicy(), null, listener, null,
-                globalCheckpoint::get, new HierarchyCircuitBreakerService(defaultSettings.getSettings(),
+            EngineConfig config = config(defaultSettings, store, createTempDir(),
+                NoMergePolicy.INSTANCE, // we don't merge we want no background merges to happen to ensure we have consistent breaker stats
+                null, listener, null, globalCheckpoint::get, new HierarchyCircuitBreakerService(defaultSettings.getSettings(),
                     new ClusterSettings(defaultSettings.getNodeSettings(), ClusterSettings.BUILT_IN_CLUSTER_SETTINGS)));
             CircuitBreaker breaker = config.getCircuitBreakerService().getBreaker(CircuitBreaker.ACCOUNTING);
             long expectedUse;
             try (InternalEngine engine = createEngine(config)) {
                 addDocuments(globalCheckpoint, engine);
-                engine.refresh("test"); // pull the reader
+                engine.flush(false, true); // first flush to make sure we have a commit that we open in the frozen engine blow.
+                engine.refresh("test"); // pull the reader to account for RAM in the breaker.
                 expectedUse = breaker.getUsed();
-                engine.flushAndClose();
             }
             assertTrue(expectedUse > 0);
             assertEquals(0, breaker.getUsed());