Paypal - Customer able to do payment using Sendbox credentials on live environment and paypal taxes not applied on payment page

Steps to Reproduce :
============================

1. Admin -> Configuration -> Settings -> Payment Methods -> Set paypal live environment
2. Login into customer
3. Purchase product or top-up account using Sendbox credentials
4. Check payment status

============================
Exp. Result : In live environment, sandbox credentials should not work.

========================================================

Steps to Reproduce :
============================

1. Admin -> Configuration -> Settings -> Payment Methods -> Set paypal Taxes % -> Save configuration
2. login in customer
3. Purchase product or top-up account using Sendbox credentials
4. Check Taxes applied or not

============================
Exp. Result : Paypal Taxes should be apply on customer account.