There are many legitimate reasons why you would still want the payments to go to the gateway when using test gateways. A big one is that we wouldn't be able to test issues customers reports or work on bug fixes/features with out it, it is extensively used through our development process and by customers who provide development and support to other users of Commerce.
The thing is, the constant will change most gateways to send data to their sandbox/test environments instead of their live environment but Stripe does not work in the same way. Stripe relies on the API Key provided to determine if the data is for a live transaction or a test transaction. If you're using a live API key on your test site then Stripe is going to interpret the data as live data.
TLDR, Generate a test api key and use that on your test site instead.