Admin shipping method spec: Wait for flash message

Otherwise, the current path might not be what we expect yet.

(cherry picked from commit 1d42a7c)

Author: mamhoff

backend/spec/features/admin/configuration/shipping_methods_spec.rb

@@ -35,6 +35,7 @@
       end
 
       click_on "Create"
+      expect(page).to have_content("Shipping Method \"bullock cart\" has been successfully created!")
       expect(current_path).to eql(spree.edit_admin_shipping_method_path(Spree::ShippingMethod.last))
 
       visit spree.new_admin_shipping_method_path