Consolidate read/write-Pixels testing code

Change-Id: I853f8f747ed0040333473fbc722cabac84e6ac83
Reviewed-on: https://skia-review.googlesource.com/7560
Commit-Queue: Robert Phillips <robertphillips@google.com>
Reviewed-by: Brian Osman <brianosman@google.com>
6 files changed