How to build a testing environment for domain name registration service

Different registrars have different requirements for getting developer access, which is what you'll need. I don't think Virtualmin's domain registrar plugin currently supports Verisign (I believe it supports Gandi, Namecheap, and Register.com, but maybe there are some third party options out there).

If you're planning to actually become a top-level registrar yourself, you'll need to build an awful lot of infrastructure. Most companies start by using OpenSRS or reselling some other providers service, while building their own infrastructure. Virtualmin and Cloudmin do not attempt to provide any registrar services directly...it merely ties into existing registrars using their APIs.