Fix Flaky Test PersistentTasksExecutorFullRestartIT.testFullClusterRestart (opensearch-project#13350)

Signed-off-by: Pranshu Shukla <pranshushukla06@gmail.com>

Author: Pranshu-S

server/src/internalClusterTest/java/org/opensearch/persistent/PersistentTasksExecutorFullRestartIT.java

@@ -43,6 +43,7 @@
 import java.util.Collection;
 import java.util.Collections;
 import java.util.List;
+import java.util.concurrent.TimeUnit;
 
 import static org.hamcrest.Matchers.empty;
 import static org.hamcrest.Matchers.equalTo;
@@ -129,7 +130,7 @@ public void testFullClusterRestart() throws Exception {
                     .custom(PersistentTasksCustomMetadata.TYPE)).tasks(),
                 empty()
             );
-        });
+        }, 20, TimeUnit.SECONDS);
 
     }
 }