Set testnet anti-replay sunset to be in the future

Summary:
The mainnet height is a long time in the past on testnet so
anti-abc chain replay was disabled.

Test Plan:
Tested electrum anti-replay txs are now not accepted on ABC fork on testnet;
they used to be before

Reviewers: deadalnix, #bitcoin_abc

Reviewed By: deadalnix, #bitcoin_abc

Differential Revision: https://reviews.bitcoinabc.org/D357