Add concrete (polkadot) BEEFY signature tests

[tomusdrw]

Example

	#[test]
	fn beefy_test() {
		let signature = hex!(
			"c557781c98f8ba8f8025accf037b83e4861101712e7cf316474f7d2f05f1894f55ea99db6a735d004e226b3aa5d3537b301bfedf1b4a095110eb7afc05b0563700"
			// "bdb33475db0157a6f93d12b72c6ed78e3ec6fd5f09cdf79d01da11603fa594bd73240f9823995c71ee6b62062603cca9fd7184d0c249e821f928db010af671a100"
		).to_vec();
		let signature: BeefySignature = signature.try_into().unwrap();

		let payload = H256::from_slice(&hex!(
			"a148e08efa4936efd7fb46109a917ee24ded43e230625da90a123929e1f9b8a5"
		));
		let block_number: BlockNumber = 1366;
		let validator_set_id = 0;
		let commitment = beefy_primitives::Commitment {
			payload,
			block_number,
			validator_set_id,
		};
		let message = commitment.encode();

		println!("Message: {}", hex::encode(&message));
		println!("Hashed: {}", hex::encode(sp_core::blake2_256(&message)));

		let untyped: <BeefySignature as sp_application_crypto::AppKey>::UntypedGeneric = signature.into();
		let public = untyped.recover(message).unwrap();
		println!("Public: {:?}", public);
		let public = secp256k1::PublicKey::parse_compressed(&public.0).unwrap();
		println!("Uncompressed: {:?}", hex::encode(public.serialize()));

		println!("Keccak256: {:?}", hex::encode(
			sp_core::keccak_256(&public.serialize()[1..])
		));

                // TODO: some assertions
	}

Currently it's hard to get concrete test data for signed commitments that would make it easy to compare with Ethereum implementation.
I imagine this would be added to the light client tests.