[SECP256K1] Add tests to exercise lambda split near bounds

Summary:
Partial backport 5/11 of secp256k1 PR830:
https://github.com/bitcoin-core/secp256k1/pull/830/commits/9d2f2b44d895509e8c4e7831fa917f13fa69f054

Depends on D8040.

Test Plan:

ninja check-secp256k1

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D8041