Fix view spec by adding mocks to helper methods

spec/views/spree/admin/orders/invoice.html.haml_spec.rb

@@ -21,6 +21,10 @@ describe "spree/admin/orders/invoice.html.haml" do
 
   before do
     assign(:order, order)
+    allow(view).to receive_messages checkout_adjustments_for: [],
+                                    display_checkout_tax_total: '10',
+                                    display_checkout_total_less_tax: '8',
+                                    outstanding_balance_label: 'Outstanding Balance'
   end
 
   it "displays the customer code" do