generate new test that did already exist & update all module snippets with declaration

test/symmetricDifference/symmetricDifference.js

@@ -1,4 +1,4 @@
-module.exports = (a, b) => {
+module.exports = symmetricDifference = (a, b) => {
 const sA = new Set(a),
 sB = new Set(b);
 return [...a.filter(x => !sB.has(x)), ...b.filter(x => !sA.has(x))];