Add comments indicating "Schnorr" in Schnorr-related script tests.

Summary: It's easy to get lost in the script_tests.cpp file, this helps keep track of what you're looking at.

Test Plan:
Read the comments to see that they make sense.
make check

Reviewers: markblundeberg, #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D3130