Add unit tests for edge cases with delta=1/2 variant of divsteps
Add unit tests for edge cases with delta=1/2 variant of divsteps

afk11/secp256k1,bitcoin/secp256k1,ElementsProject/secp256k1-zkp,afk11/secp256k1,bitcoin-core/secp256k1,bitcoin/secp256k1,bitcoin/secp256k1,bitcoin/secp256k1,bitcoin/secp256k1,bitcoin-core/secp256k1,afk11/secp256k1,afk11/secp256k1,peterdettman/secp256k1,peterdettman/secp256k1,ElementsProject/secp256k1-zkp,afk11/secp256k1