Can I buy the Xbox one x for testing at Saturn and then return it?
Online yes
Yes, that should work. You have a 14-day return policy.
Simply say you didn't like the product. Treat well, don't break anything, and just keep the original packaging.
Saturn does not have to take back the goods, you would have the opportunity to test them in the shop before buying, then you would rather order them online
Only online or because of me at doorstep stores. There's no statutory right of withdrawal in retail, even if Saturn is generally very accommodating, it remains goodwill and can also be refused.