[schnorr functional test] remove ComparisonTestFramework dependency

Summary:
This switches the Schnorr test to not rely on comptool (which is
deprecated), and takes advantage of the newly backported P2PDataStore
features. bootstrap_p2p and reconnect_p2p are borrowed from
p2p_invalid_tx.py (introduced D3527).

This nicely removes the need to rely on -whitelist for rejected
blocks, instead we can send them and get "banned" (disconnected), and
then reconnect.

I've also added some logging to the test, while I'm at it.

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

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

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D3601