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();

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

GuessTheNewNumberSolver.sol

pragma solidity ^0.4.21;

import "./GuessTheNewNumberChallenge.sol";

contract GuessTheNewNumberSolver {
    function isComplete(address _challenge) public view returns(bool) {
        GuessTheNewNumberChallenge challenge = GuessTheNewNumberChallenge(_challenge);
        return challenge.isComplete();
    }
}