Modernize the downloadFile test helper function

The test helper code largely predates the introduction of modern
JavaScript features and should be refactored to improve readability.
In particular callbacks make the code harder to understand and maintain.

This commit:

- replaces the callback argument with returning a promise;
- uses `const` instead of `var`;
- uses arrow functions for shorter code;
- uses template strings for shorter string formatting code;
- uses `Array.includes` for shorter response code checking code.

test/downloadutils.mjs

@@ -34,48 +34,47 @@ function rewriteWebArchiveUrl(url) {
   return url;
 }
 
-function downloadFile(file, url, callback, redirects) {
+function downloadFile(file, url, redirects = 0) {
   url = rewriteWebArchiveUrl(url);
+  const protocol = /^https:\/\//.test(url) ? https : http;
 
-  var protocol = /^https:\/\//.test(url) ? https : http;
-  protocol
-    .get(url, function (response) {
-      if (
-        response.statusCode === 301 ||
-        response.statusCode === 302 ||
-        response.statusCode === 307 ||
-        response.statusCode === 308
-      ) {
-        if (redirects > 10) {
-          callback("Too many redirects");
+  return new Promise((resolve, reject) => {
+    protocol
+      .get(url, async function (response) {
+        if ([301, 302, 307, 308].includes(response.statusCode)) {
+          if (redirects > 10) {
+            reject(new Error("Too many redirects"));
+            return;
+          }
+          const redirectTo = urlResolve(url, response.headers.location);
+          try {
+            await downloadFile(file, redirectTo, ++redirects);
+            resolve();
+          } catch (ex) {
+            reject(ex);
+          }
+          return;
         }
-        var redirectTo = response.headers.location;
-        redirectTo = urlResolve(url, redirectTo);
-        downloadFile(file, redirectTo, callback, (redirects || 0) + 1);
-        return;
-      }
 
-      if (response.statusCode !== 200) {
-        callback("HTTP " + response.statusCode);
-        return;
-      }
-      var stream = fs.createWriteStream(file);
-      stream.on("error", function (err) {
-        callback(err);
-      });
-      response.pipe(stream);
-      stream.on("finish", function () {
-        stream.end();
-        callback();
-      });
-    })
-    .on("error", function (err) {
-      callback(err);
-    });
+        if (response.statusCode !== 200) {
+          reject(new Error(`HTTP ${response.statusCode}`));
+          return;
+        }
+
+        const stream = fs.createWriteStream(file);
+        stream.on("error", error => reject(error));
+        stream.on("finish", () => {
+          stream.end();
+          resolve();
+        });
+        response.pipe(stream);
+      })
+      .on("error", error => reject(error));
+  });
 }
 
 function downloadManifestFiles(manifest, callback) {
-  function downloadNext() {
+  async function downloadNext() {
     if (i >= links.length) {
       callback();
       return;
@@ -83,15 +82,15 @@ function downloadManifestFiles(manifest, callback) {
     var file = links[i].file;
     var url = links[i].url;
     console.log("Downloading " + url + " to " + file + "...");
-    downloadFile(file, url, function (err) {
-      if (err) {
-        console.error("Error during downloading of " + url + ": " + err);
-        fs.writeFileSync(file, ""); // making it empty file
-        fs.writeFileSync(file + ".error", err);
-      }
-      i++;
-      downloadNext();
-    });
+    try {
+      await downloadFile(file, url);
+    } catch (ex) {
+      console.error(`Error during downloading of ${url}: ${ex}`);
+      fs.writeFileSync(file, ""); // making it empty file
+      fs.writeFileSync(`${file}.error`, ex);
+    }
+    i++;
+    downloadNext();
   }
 
   var links = manifest