Fix unpersisted changes in checkout controller specs

The test setup left the object with unsaved changes, which doesn't work nicely with the new order locking added recently on the edit action.

Fixes ~5 specs including:

5) CheckoutController redirection to cart and stripe redirects when some items are out of stock
     Failure/Error:
       order.with_lock do
         lock_variants_of(order)
         yield
       end

     RuntimeError:
       Locking a record with unpersisted changes is not supported. Use `save` to persist the changes, or `reload` to discard them explicitly.
     # ./app/services/current_order_locker.rb:22:in `lock_order_and_variants'
     # ./app/services/current_order_locker.rb:11:in `around'
     # ./spec/controllers/checkout_controller_spec.rb:57:in `block (3 levels) in <top (required)>'

spec/controllers/checkout_controller_spec.rb

@@ -46,7 +46,7 @@ describe CheckoutController, type: :controller do
     before do
       allow(controller).to receive(:current_order).and_return(order)
       allow(order).to receive(:distributor).and_return(distributor)
-      order.order_cycle = order_cycle
+      order.update(order_cycle: order_cycle)
 
       allow(OrderCycleDistributedVariants).to receive(:new).and_return(order_cycle_distributed_variants)
     end