[schnorr functional test] simplify funding logic

Summary:
This simplifies and clarifies the funding setup further; no need to
keep track of which exact coinbases are being spent.

Depends on D3615

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/D3616