[schnorr functional test] change block setup logic

Summary:
This gets rid of the time.time() dependency, allowing the node to set
the chain time naturally using the generate RPC. Likewise this gets
rid of the arbitrary-block-numbering and auto-tip-movement system and
moves to a more direct form of tracking.

At the same time, we allow setup_clean_chain to be False, in which
case the generated coinbases will be smaller, so the funding needs to
be tweaked slightly.

Depends on D3601

Test Plan: ./test/functional/test_runner.py abc-schnorr.py

Reviewers: #bitcoin_abc, Mengerian, Fabien, jasonbcox, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D3615