fix mistake in test framework's schnorr signing module

Summary:
Python feature: you can keep parts of your program as pseudocode if they
never actually run (or only get run with 1/2^128 probability like here).

(thanks @Fabien for picking this up with linter)

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

Reviewers: Fabien, #bitcoin_abc, deadalnix

Reviewed By: Fabien, #bitcoin_abc

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D3563