Add post-upgrade testnet checkpoint

Summary:
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).

Test Plan:
make check
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