Support splitting exhaustive tests across cores

Summary:
This is a partial backport of secp256k1 PR808 : https://github.com/bitcoin-core/secp256k1/pull/808/commits/39f67dd072fc44c7c0d27b95610ba8912de56db5

Depends on D7663

Test Plan:

ninja check-secp256k1

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D7664