[9.0] Fix MergeWithFailureIT (elastic#135447)

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

Closes elastic#135443

Author: tlrx

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

@@ -359,7 +359,7 @@ public void testFailedMergeDeadlock() throws Exception {
         ensureRed(indexName);
 
         // verify that the shard store is effectively closed
-        assertTrue(plugin.shardStoreClosedListener.isDone());
+        safeGet(plugin.shardStoreClosedListener);
 
         if (closingThread != null) {
             closingThread.join();