Fix JavaScript error for ExternalUserStatistics

Previously, an exercise without visible files was used. While this worked for the test, it was semantically problematic, since the resulting submissions also lacked files.

spec/features/external_user_statistics_spec.rb

@@ -4,7 +4,7 @@ require 'rails_helper'
 
 RSpec.describe 'ExternalUserStatistics', :js do
   let(:learner) { create(:external_user) }
-  let(:exercise) { create(:dummy, user:) }
+  let(:exercise) { create(:fibonacci, user:) }
   let(:study_group) { create(:study_group) }
   let(:password) { 'password123456' }