[SECP256K1] Check correctness of lambda split without -DVERIFY
Summary:
The VERIFY macro turns on various paranoid consistency checks, but the complete functionality should still be tested without it. This also adds a couple of static test points for extremely small split inputs/outputs. The existing bounds vectors already check extremely large outputs.
Partial backport 7/11 of secp256k1 PR830:
https://github.com/bitcoin-core/secp256k1/pull/830/commits/ebad8414b0e68041568d0b5ebe0bd395dbfbed9e
Depends on D8042.
Test Plan:
ninja check-secp256k1
Reviewers: #bitcoin_abc, deadalnix
Reviewed By: #bitcoin_abc, deadalnix
Differential Revision: https://reviews.bitcoinabc.org/D8043