Add exhaustive test for extrakeys and schnorrsig

Summary:
This is a partial backport of secp256k1 PR808 : https://github.com/bitcoin-core/secp256k1/pull/808/commits/8b7dcdd955a4f57174f478e36bdae5b84784fb9c

Depends on D7667

Test Plan:

ninja check-secp256k1

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D7668