[secp256k1] Extend libsecp256k1's ctime test to check schnorr signatures

Summary: This comes with a set of refactoring to the schnorr signature code to avoid tripping the checks. Nothing really bad was discovered, but now we know that nothign really bad will ever be introduced.

Test Plan:

libtool --mode=execute valgrind src/secp256k1/valgrind_ctime_test

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D5597