Fix abc-finalize-block.py header propagation issue

Summary:
The wait_for_invalid_block success is dependent of the header propagation.
It assumes that the last block generated by alt_node will become the tip
of node, which is not the case if previous invalid headers are received.

Also add ascii art in comments

Test Plan:

./test/functional/test_runner.py abc-finalize-block

Reviewers: #bitcoin_abc, deadalnix, schancel, jasonbcox

Reviewed By: #bitcoin_abc, deadalnix, schancel, jasonbcox

Subscribers: jasonbcox, schancel, deadalnix, teamcity

Differential Revision: https://reviews.bitcoinabc.org/D2130