Fix failing js tests

The test suits need window.mailpoet_feature_flags to be defined
in case the feature flags are checked in the code

[MAILPOET-5193]

mailpoet/tests/javascript_newsletter_editor/mochaTestHelper.js

@@ -583,3 +583,4 @@ global.templates = {
   ),
 };
 global.window.templates = global.templates;
+global.window.mailpoet_feature_flags = { brand_templates: true };