valgrind_ctime_test: Add schnorrsig_sign

Summary:
This is a partial backport of secp256k1 PR558 : https://github.com/bitcoin-core/secp256k1/pull/558/commits/f431b3f28ac95a3645ad5a6dc96b878fa30a1de3

Depends on D7650

Test Plan:

ninja check-secp256k1

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D7651