Preparations for more testchains

Summary:

• Testchains: Generic selection with -chain=<str> in addition of -testnet and -regtest
• Testchains: Qt: Simplify network/chain styles

This is a backport of Core PR16680

Depends on D5887

Test Plan:

make check

Run bitcoin-qt on win64

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D5888