Add tests for pre_g tables.
Add tests for pre_g tables.

We check that the static table entries are all correct.

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