Fix flaky add to cart actions in feature test

Clicking links and then immediately navigating to another page before
the previous link resolves creates a race condition which causes flaky
tests.

Author: AlistairNorman

templates/spec/system/promotion_code_invalidation_spec.rb

@@ -19,10 +19,13 @@
 
     visit products_path
     click_link "DL-44"
+    expect(page).to have_content("As seen on TV!")
     click_button "Add To Cart"
+    expect(page).to have_content("Shopping Cart")
 
     visit products_path
     click_link "E-11"
+    expect(page).to have_content("As seen on TV!")
     click_button "Add To Cart"
   end