Add a regtest() constructor method to Network

[mpguerra]

Motivation

This has been requested, and could be useful for testing.

Possible Design

Add a method that returns a Network::Testnet with Regtest parameters.

[teor2345]

I don't think we should make this change add a Regtest variant to Network, I think we should add fields to the existing Testnet enum variant instead, like @arya2 and I talked about.

[arya2]

This is blocked on adding some of the hard-coded parameters that need to be changed for RegTest to the Network::Testnet variant in #7845

[mpguerra]

Does the issue title accurately reflect what we are going to do here?