Add spec for registration link on Spree pages

The `registration_path` helper resolves to `/signup` in Spree
controllers due to spree_auth_device > config > routes.rb.
We worked around that in:
https://github.com/openfoodfoundation/openfoodnetwork/pull/3174

Here we add a spec for this so that we can test more easily if we can
remove that workaround or detect it's accidental removal.

spec/features/consumer/shopping/cart_spec.rb

@@ -20,6 +20,17 @@ feature "full-page cart", js: true do
       set_order order
     end
 
+    it "shows the right registration link" do
+      # We have an issue with the registration link within Spree controllers.
+      # The `registration_path` helper resolves to `/signup` due to
+      # spree_auth_device > config > routes.rb. This spec verifies that we have
+      # a solution to that problem.
+      add_product_to_cart order, product_with_fee
+      visit spree.cart_path
+      register_page = window_opened_by { click_link "Register here" }
+      within_window(register_page) { expect(page).to have_content "Welcome" }
+    end
+
     describe "product description" do
       it "does not link to the product page" do
         add_product_to_cart order, product_with_fee, quantity: 2