Extend test coverage for bls12-381
Extend test coverage for bls12-381

- test to_encoded_string,
- test CryptoMaterial::WrongLengthEncoding
- test genesis

Closes: #2680
Approved by: kchalkias

aptos-labs/aptos-core,libra/libra,libra/libra,aptos-labs/aptos-core,aptos-labs/aptos-core,aptos-labs/aptos-core,libra/libra,libra/libra,libra/libra,aptos-labs/aptos-core,aptos-labs/aptos-core,aptos-labs/aptos-core