cli: replace testnet with chain and return network name as per BIP70.

Summary: Backport of Core PR15566

Test Plan:
ninja && ninja check-functional

Install and test:
bitcoin-cli -getinfo

Reviewers: O1 Bitcoin ABC, #bitcoin_abc, Fabien

Reviewed By: O1 Bitcoin ABC, #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D7734