[CI] Fix MergeWithFailureIT (elastic#135447)

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

Closes elastic#135443

Author: tlrx

muted-tests.yml

@@ -609,9 +609,6 @@ tests:
 - class: org.elasticsearch.gradle.TestClustersPluginFuncTest
   method: override jdk usage via ES_JAVA_HOME for known jdk os incompatibilities
   issue: https://github.com/elastic/elasticsearch/issues/135413
-- class: org.elasticsearch.index.engine.MergeWithFailureIT
-  method: testFailedMergeDeadlock
-  issue: https://github.com/elastic/elasticsearch/issues/135443
 - class: org.elasticsearch.xpack.esql.qa.single_node.EsqlSpecIT
   method: test {csv-spec:spatial_shapes.ConvertCartesianShapeFromStringParseError}
   issue: https://github.com/elastic/elasticsearch/issues/135455

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

@@ -321,7 +321,7 @@ 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();