Include 4f variants of random gradients during testing

BUG=skia:

GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=3538

Change-Id: Ieee6e49cb830b6aab87b0ecd7865c65ffb90dfe8
Reviewed-on: https://skia-review.googlesource.com/3538
Reviewed-by: Florin Malita <fmalita@chromium.org>
Commit-Queue: Brian Osman <brianosman@google.com>
6 files changed