test: include derivation test when private key has leading zeros

[braydonf]

• Tests derivation implementation to bip32 spec when private key has leading zeros as it's possible that the private key could be represented with less than 32 bytes.
• Cross tested against:
  • libbtc at https://github.com/braydonf/libbtc/commits/cross-ref
  • hdkey at test: include derivation test when private key has leading zeros cryptocoinjs/hdkey#9
  • bcoin at test: include derivation test when private key has leading zeros bcoin-org/bcoin#84
  • bitcoinjs-lib at test: include derivation test when private key has leading zeros bitcoinjs/bitcoinjs-lib#673
  • bitcore-lib at Fix implementation of hd derivation to be bip32 compliant bitpay/bitcore-lib#97

[ryanxcharles]

Awesome, thanks for your PR! We will merge this as soon as we figure out how to reconcile our private internal version of this repo with the external version.