Add Gray test color images to bots

BUG=skia:

Change-Id: I70a92a9f6355f0fbdfd64b528d10bd0c25a9c0a0
Reviewed-on: https://skia-review.googlesource.com/5864
Commit-Queue: Matt Sarett <msarett@google.com>
Reviewed-by: Robert Aftias <raftias@google.com>
tree: 03120085e3b2f1827dc09d682415287bc57f0466
  1. recipe_modules/
  2. recipes.py
  3. recipes/