Make group order purely compile-time in exhaustive tests

Summary: This is a partial backport of secp256k1 PR808 : https://github.com/bitcoin-core/secp256k1/pull/808/commits/be317915436909573733afe3972a9abdee9357f7

Test Plan:

ninja check-secp256k1

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D7654