change countFiles method to also use fs-Extra

alfa-client/apps/alfa-e2e/src/support/cypress-tasks.ts

 import { Db, Long, MongoClient, ObjectId } from 'mongodb';
-const fs = require('fs');
 const fsExtra = require('fs-extra');
 
 const Binary = require('mongodb').Binary;
@@ -317,15 +316,12 @@ function dropCollections(databaseUrl, databaseName, collections) {
   });
 }
 
-function countFiles(folderName: string): Promise<number> {
-  return new Promise((resolve, reject) => {
-    fs.readdir(folderName, (err, files) => {
-      if (err) {
-        console.error(err);
-        return reject(err);
-      }
-      resolve(files.length);
-    });
+function countFiles(folderName) {
+  return fsExtra
+    .readdir(folderName)
+    .then((files) => files.length)
+    .catch((err) => {
+      throw new Error(`Counting files failed: ${err.message}`);
     });
 }