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
  • yours-bitcoin at test: include derivation test when private key has leading zeros moneybutton/yours-bitcoin#48
  • bitcore-lib at Fix implementation of hd derivation to be bip32 compliant bitpay/bitcore-lib#97

[dcousens]

Probably would add this as a test fixture instead, but sure? Was there any reason why this might not work?

[braydonf]

All of the tests will currently pass if the 32 is removed from line
https://github.com/bitcoinjs/bitcoinjs-lib/blob/master/src/hdnode.js#L217, and changed into:

this.keyPair.d.toBuffer().copy(data, 1)

Instead of:

this.keyPair.d.toBuffer(32).copy(data, 1)

So this test covers that case.

There may be one more at, that isn't covered: https://github.com/bitcoinjs/bitcoinjs-lib/blob/master/src/hdnode.js#L193

Most of the fixtures have a seed, the seed is unknown here.

[dcousens]

@braydonf just so we didn't need to change our testing fixtures dramatically with more branching (though, my refactoring did that anyway), I generated a fixture that had 5 leading leading zeros here

[dcousens]

Whatever, we'll merge both