Fix order-dependent DesignManagement::SaveDesignsService spec

This spec uses `build_stubbed`, which uses IDs starting at 1000 and
incrementing on their own internal counter. It appears that we had a
combination of specs where this ID matched an actual database ID of a
user with access (probably `developer` in this spec file), which is
unlikely but not impossible.

By using `non_existing_record_id` as the ID for this stubbed record, we
ensure that can't happen.

spec/services/design_management/save_designs_service_spec.rb

@@ -309,7 +309,7 @@ RSpec.describe DesignManagement::SaveDesignsService do
       end
 
       context 'when the user is not allowed to upload designs' do
-        let(:user) { build_stubbed(:user) }
+        let(:user) { build_stubbed(:user, id: non_existing_record_id) }
 
         it_behaves_like 'a service error'
       end