Use EEU_FEATURES during feature stubbing

This saves work during stubbing, and is just a lot clearer.

ee/spec/support/helpers/ee/license_helpers.rb

@@ -15,8 +15,8 @@ module EE
 
     prepended do
       def stub_licensed_features(features)
-        existing_features = License::FEATURES_BY_PLAN.values.flatten.map(&:to_sym)
-        missing_features = features.keys.map(&:to_sym) - existing_features
+        # EEU_FEATURES contains all the features we know about
+        missing_features = features.keys.map(&:to_sym) - License::EEU_FEATURES
 
         if missing_features.any?
           subject = missing_features.join(', ')