As a follow up to commit "8d6af086 Use dedicated ALPHA passthrough shader",
this change adds a regression test that verifies RGB channels are set to
zero when performing an ALPHA copy.

BUG=angleproject:2239
Change-Id: Icc08deb523c388a0f2cbdcadd1eec69a97fbe136
Reviewed-on: https://chromium-review.googlesource.com/774579
Commit-Queue: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>