Force flush in FrozenEngine#testSearchers (#51635)

We need to force flush to make the last commit safe; otherwise, we might 
fail to open FrozenEngine. Note that we force flush before closing a
shard.

Closes #51620

Author: dnhatn

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

@@ -342,7 +342,9 @@ public void testSearchers() throws Exception {
                 applyOperations(engine, generateHistoryOnReplica(between(10, 1000), false, randomBoolean(), randomBoolean()));
                 globalCheckpoint.set(engine.getProcessedLocalCheckpoint());
                 engine.syncTranslog();
-                engine.flush();
+                // We need to force flush to make the last commit a safe commit; otherwise, we might fail to open ReadOnlyEngine
+                // See TransportVerifyShardBeforeCloseAction#executeShardOperation
+                engine.flush(true, true);
                 engine.refresh("test");
                 try (Engine.Searcher searcher = engine.acquireSearcher("test")) {
                     totalDocs = searcher.search(new MatchAllDocsQuery(), Integer.MAX_VALUE).scoreDocs.length;