Fix testRestartAfterCompletion (elastic#63211)

We need to complete the search before closing the iterator, which 
internally closes the point in time; otherwise, the search will fail
with a missing context error.

Closes elastic#62451

x-pack/plugin/async-search/src/internalClusterTest/java/org/elasticsearch/xpack/search/AsyncSearchActionIT.java

@@ -179,6 +179,9 @@ public void testRestartAfterCompletion() throws Exception {
         try (SearchResponseIterator it =
                  assertBlockingIterator(indexName, numShards, new SearchSourceBuilder(), 0, 2)) {
             initial = it.next();
+            while (it.hasNext()) {
+                it.next();
+            }
         }
         ensureTaskCompletion(initial.getId());
         restartTaskNode(initial.getId(), indexName);