Load referenced lib class for independent spec run

This spec file tried to access a class without loading the code. That
worked in usual spec runs when other specs were run first which
loaded the whole lib directory.

spec/serializers/api/admin/subscription_customer_serializer_spec.rb

@@ -1,6 +1,7 @@
 # frozen_string_literal: true
 
 require 'spec_helper'
+require 'open_food_network/address_finder'
 
 describe Api::Admin::SubscriptionCustomerSerializer do
   let(:address) { build(:address) }