Update chainparams for 0.19.1 release

Summary: See title. Updated testnet params to be the block before the upgraded-testnet fork.

Test Plan: Verify on your own node(s)

Reviewers: deadalnix, Fabien, Mengerian, #bitcoin_abc

Reviewed By: Fabien, Mengerian, #bitcoin_abc

Subscribers: teamcity, schancel

Differential Revision: https://reviews.bitcoinabc.org/D2669