[piexwasm] test.js: Promise.all write the images to the file system

Use Promise.all to write the test image files to the file system. This
allows the writes to happen in parallel. Test setup time before: 3 sec
and after: 2.2 sec.

The setup time is HTML fetch-bound, not I/O bound, and parallel writes
only improve the I/O (write speed) component of the setup time.

Bug: 935285
Change-Id: I872a111477c6fff1c692d45ed3b93d6b68d6d476
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1562810Reviewed-by: Luciano Pacheco <lucmult@chromium.org>
Commit-Queue: Noel Gordon <noel@chromium.org>
Auto-Submit: Noel Gordon <noel@chromium.org>
Cr-Commit-Position: refs/heads/master@{#649740}

ui/file_manager/image_loader/piex/tests.js

@@ -95,11 +95,11 @@ const puppeteer = require('puppeteer');
     return window.createFileSystem(length);
   }, images.length);
 
-  for (let i = 0; i < images.length; ++i) {
-    await page.evaluate((image) => {
+  await Promise.all(images.map((image) => {
+    return page.evaluate((image) => {
       return window.writeToFileSystem(image);
-    }, images[i]);
-  }
+    }, image);
+  }));
 
   await page.evaluate(() => {
     window.testTime = 0;