[secp256k1] exhaustive_tests: Fix with ecmult_gen table with custom generator

Summary:
This enables the secp256k1 exhaustive tests again.

This is a partial backport of core#988
https://github.com/bitcoin-core/secp256k1/pull/988/commits/5106226991117da78f85ca88b7ea66c2ac8fe0b3

Depends on D19387

Test Plan: ninja check-secp256k1

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D19388