Add post-upgrade testnet checkpoint
Post-upgrade checkpoint will facilitate simplifying
upgrade activation code.
This checkpoint is block height 1303885, which correponds to the first
block with new rules activated (second block with MTP > activation).
Did an IBD on testnet, synced to the expected chaintip.
Check against public blockchain explorers.
Reviewers: deadalnix, jasonbcox, Fabien, markblundeberg, #bitcoin_abc
Reviewed By: Fabien, markblundeberg, #bitcoin_abc
Maniphest Tasks: T653
Differential Revision: https://reviews.bitcoinabc.org/D3149