finish spawn_pending test

simplify conditions to check, fix next url, add missing ready signal to spawn_pending.html

need slow_spawn to be even slower

share/jupyterhub/templates/spawn_pending.html

@@ -87,6 +87,8 @@ require(["jquery"], function ($) {
     }
   };
 
+  // signal that page has finished loading (mostly for tests)
+  window._jupyterhub_page_loaded = true;
 });
 </script>
 {% endblock %}