browser/ember: Rethrow errors in testing mode (#1696)

see emberjs/ember-qunit#304

Author: Turbo87

packages/browser/src/integrations/pluggable/ember.ts

@@ -49,6 +49,8 @@ export class Ember implements Integration {
 
       if (typeof oldOnError === 'function') {
         oldOnError.call(this.Ember, error);
+      } else if (this.Ember.testing) {
+        throw error;
       }
     };