This warning could pop up whenever an OpenGL app would write
to a particular output and not bind an attachment. This is
valid in OpenGL. Also it could happen when writing to
gl_FragData instead of gl_FragColor. Since it's hard to fix
every usage we can just suppress the warning.

Also adds a way to change test platform warnings into errors.

Bug: angleproject:2866
Change-Id: I9793f58121ac848d74d6b0131e79ebab2c70f45c
Reviewed-on: https://chromium-review.googlesource.com/c/1462057
Commit-Queue: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>