Also add a warning on top of the UnrollFlatten test that it works only
when HLSL is enabled.

BUG=angleproject:1140

Change-Id: Ide961bbed70dc9ed2a604f13273e4209b1dec29b
Reviewed-on: https://chromium-review.googlesource.com/295125Reviewed-by: Zhenyao Mo <zmo@chromium.org>
Tested-by: Corentin Wallez <cwallez@chromium.org>