Poseidon Hardcoded Tests

[GhostOfGauss]

These currently are for the Bls12-381 curve and should be updated to Bn254.

In plugins::arkworks::poseidon::test there should be a mod hash with this test:

// mod constants {
//     use crate::poseidon::config::Spec2;
//     use openzl_crypto::poseidon::constants::Constants;

//     // TODO: After upgrading to new Poseidon, we have to enable these tests.
//     /// Tests if the specifications match the known constant values.
//     #[cfg(feature = "std")] // TODO: How do I get the test to enforce this feature?
//     #[test]
//     fn specifications_match_known_values() {
//         assert_eq!(
//             Constants::from_arity(2),
//             Constants::from_specification::<Spec2>()
//         );
//         assert_eq!(
//             Constants::from_arity(4),
//             Constants::from_specification::<Spec2>()
//         );
//     }
// }

and a mod hash with this test

mod hash {
    // TODO: Reconstruct hardcoded test with current domain tag.
    // /// Tests if [`Poseidon2`](crate::config::Poseidon2) matches hardcoded sage outputs.
    // #[test]
    // fn poseidon_hash_matches_known_values() {
    //     let hasher = Spec2::gen(&mut OsRng);
    //     let inputs = [&Fp(field_new!(Fr, "1")), &Fp(field_new!(Fr, "2"))];
    //     assert_eq!(
    //         hasher.hash_untruncated(inputs, &mut ()),
    //         include!("permutation_hardcoded_test/width3") // Why doesn't this work?
    //     );
    // }
}

[GhostOfGauss]

Also plugins::arkworks::poseidon::config::test can have a test showing that the constants specified in the module are secure