DEV: Fix a deprecated `ember` import (#9903)

See the following for `Ember.testing` and `@ember/debug isTesting`:

* 8cf29959f7/packages/ember/index.js (L372-L376)
* 8cf29959f7/packages/%40ember/debug/lib/testing.ts

app/assets/javascripts/discourse-loader.js

@@ -29,6 +29,7 @@ var define, requirejs;
         inject: Ember.inject.controller
       },
       "@ember/debug": {
+        isTesting: () => Ember.testing,
         warn: Ember.warn
       },
       "@ember/object": {

app/assets/javascripts/discourse/app/helpers/page-reloader.js

@@ -1,9 +1,9 @@
 import EmberObject from "@ember/object";
-import Ember from "ember";
+import { isTesting } from "@ember/debug";
 
 export default EmberObject.create({
-  reload: function() {
-    if (!Ember.testing) {
+  reload() {
+    if (!isTesting()) {
       location.reload();
     }
   }