Replace expecting method call with outcome

This is more realistic and robust. Don't mock the class under test (even
though `touch` is actually provided by Active Record).

spec/models/spree/order_spec.rb

@@ -220,8 +220,12 @@ RSpec.describe Spree::Order do
     subject(:order) { Spree::Order.create }
 
     it "should set completed_at" do
-      expect(order).to receive(:touch).with(:completed_at)
-      order.finalize!
+      expect {
+        order.finalize!
+        order.reload
+      }.to change {
+        order.completed_at
+      }.from(nil)
     end
 
     it "should sell inventory units" do