net, rpc, test, bugfix: update GetNetworkName, GetNetworksInfo, regression tests

Summary:
PR description:

Following up on the BIP155 addrv2 changes, and starting with core#19845, RPC getnetworkinfo began returning networks with empty names.

This patch:

• updates GetNetworkName() to the current Network enum
• updates getNetworksInfo() to ignore as-yet unsupported networks
• adds regression tests

This is a backport of core#20120

Test Plan: ninja all check-all

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D10720