tests: Fix test whose result is implementation-defined
tests: Fix test whose result is implementation-defined

A compiler may add struct padding and fe_cmov is not guaranteed to
preserve it.

On the way, we improve the identity check such that it covers the
VERIFY struct members.

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