Fix invalid Payment in Spree::Payment spec

While this spec will  pass, any additional attempts to save the payment
in question would fail as the Payment was not able to save on create
due to it being invalid (missing a source).

core/spec/models/spree/payment_spec.rb

@@ -668,12 +668,9 @@
     end
 
     context 'when the payment was completed but now void' do
-      let(:payment) do
-        Spree::Payment.create(
-          amount: 100,
-          order: order,
-          state: 'completed'
-        )
+      before do
+        payment.state = 'completed'
+        payment.amount = 100
       end
 
       it 'updates order payment total' do