feat: allow next challenge's seed to be a solution (#39145)

* feat: allow next challenge's seed to be a solution

curriculum/test/test-challenges.js

@@ -23,7 +23,7 @@ const {
 
 const { assert, AssertionError } = require('chai');
 const Mocha = require('mocha');
-const { flatten } = require('lodash');
+const { flatten, isEmpty } = require('lodash');
 
 const jsdom = require('jsdom');
 
@@ -51,6 +51,7 @@ const {
 } = require('../../client/src/templates/Challenges/utils/build');
 
 const { createPoly } = require('../../utils/polyvinyl');
+const { sortChallenges } = require('./utils/sort-challenges');
 
 const testEvaluator = require('../../client/config/test-evaluator').filename;
 
@@ -226,7 +227,9 @@ async function getChallenges(lang) {
         return [...challengeArray, ...flatten(challengesForBlock)];
       }, [])
   );
-  return challenges;
+  // This matches the order Gatsby uses (via a GraphQL query). Ideally both
+  // should be sourced and sorted using a single query, but we're not there yet.
+  return sortChallenges(challenges);
 }
 
 function validateBlock(challenge) {
@@ -245,7 +248,7 @@ function populateTestsForLang({ lang, challenges, meta }) {
 
   describe(`Check challenges (${lang})`, function() {
     this.timeout(5000);
-    challenges.forEach(challenge => {
+    challenges.forEach((challenge, id) => {
       const dashedBlockName = dasherize(challenge.block);
       describe(challenge.block || 'No block', function() {
         describe(challenge.title || 'No title', function() {
@@ -367,6 +370,15 @@ function populateTestsForLang({ lang, challenges, meta }) {
           });
 
           let { solutions = [] } = challenge;
+          // if there are no solutions in the challenge, it's assumed the next
+          // challenge's seed will be a solution to the current challenge.
+          // This is expected to happen in the project based curriculum.
+          if (isEmpty(solutions)) {
+            const nextChallenge = challenges[id + 1];
+            if (nextChallenge) {
+              solutions = [nextChallenge.files[0].contents];
+            }
+          }
           const noSolution = new RegExp('// solution required');
           solutions = solutions.filter(
             solution => !!solution && !noSolution.test(solution)