[piexwasm] test.html: implement file system read api

Add readFromFileSystem(): CL:1562254 implemented create and file write
file system api. Now add a read a file from the file system api.

Bug: 935285
Change-Id: I6cc9f530fbcf1f1e551cd09706f1c4bfb97b3517
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1566268
Reviewed-by: Luciano Pacheco <lucmult@chromium.org>
Reviewed-by: Alex Danilo <adanilo@chromium.org>
Commit-Queue: Noel Gordon <noel@chromium.org>
Cr-Commit-Position: refs/heads/master@{#650700}