Add a test to ensure memory isn't consumed for blocks pre-checkpoint

Summary:
This adds coverage on three test cases, where blocks are rejected and do not leak memory in the block index for all cases:

1. Forks from a new genesis block.
2. Forks from before a checkpoint that has been reached.
3. Forks at a checkpoint.

Test Plan:

make check
ninja check

Reviewers: deadalnix, Fabien, #bitcoin_abc

Reviewed By: deadalnix, Fabien, #bitcoin_abc

Differential Revision: https://reviews.bitcoinabc.org/D2729