[FIX] website_event_sale: fixed test to allow structural dom changes

It's not like it's the third time I change this test file, right ?

addons/website_event_sale/static/src/js/website.tour.event_sale.js

@@ -39,7 +39,7 @@
             },
             {
                 title:     "Process Checkout",
-                waitFor:   '#top_menu .my_cart_quantity:contains(6)',
+                waitFor:   'a:has(.my_cart_quantity):contains(6)',
                 element:   '.btn-primary:contains("Process Checkout")'
             },
             {