Adding a test for deflate_stored

This compression function is used only for copying data from an
input stream without compression.

Granted, is a user case we don't have on Chromium, but having code
coverage for it is just a matter of adding a test with compression
level set to zero.

Bug: 1032721
Change-Id: I67e84cc34bcadd99671d451a311a05897840595c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2188932


Reviewed-by: Adenilson Cavalcanti <cavalcantii@chromium.org>
Reviewed-by: Noel Gordon <noel@chromium.org>
Commit-Queue: Adenilson Cavalcanti <cavalcantii@chromium.org>
Cr-Original-Commit-Position: refs/heads/master@{#766711}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: a50938921e9fb8108bf7b5b25ff3bca95d0b75b9