[8.19] Fix MergeWithFailureIT (#135523)

Wait for the store to be closed, don't assume
it happens immediately.

Closes #135443

server/src/internalClusterTest/java/org/elasticsearch/index/engine/MergeWithFailureIT.java

@@ -353,7 +353,7 @@ public class MergeWithFailureIT extends ESIntegTestCase {
         ensureRed(indexName);
 
         // verify that the shard store is effectively closed
-        assertTrue(plugin.shardStoreClosedListener.isDone());
+        safeGet(plugin.shardStoreClosedListener);
 
         if (closingThread != null) {
             closingThread.join();