[TEST] wait for yellow before query execution

We can remove this once https://github.com/elastic/elasticsearch/pull/18759
is in.

docs/reference/search/request/sort.asciidoc

@@ -26,6 +26,8 @@ PUT /my_index
         }
     }
 }
+
+GET /_cluster/health?wait_for_status=yellow
 --------------------------------------------------
 // CONSOLE