Improve specs

Better to rely on a verifying double and reveal our intentions using
`subject` instead.

spec/controllers/spree/users_controller_spec.rb

@@ -44,7 +44,7 @@ describe Spree::UsersController, type: :controller do
     end
 
     context 'when the customer_balance feature is enabled' do
-      let(:outstanding_balance) { double(:outstanding_balance) }
+      let(:outstanding_balance) { instance_double(OutstandingBalance) }
 
       before do
         allow(OpenFoodNetwork::FeatureToggle)

spec/queries/outstanding_balance_spec.rb

@@ -3,7 +3,7 @@
 require 'spec_helper'
 
 describe OutstandingBalance do
-  let(:outstanding_balance) { described_class.new(relation) }
+  subject(:outstanding_balance) { described_class.new(relation) }
 
   describe '#statement' do
     let(:relation) { Spree::Order.none }