Consolidate read/write-Pixels testing code - take 2

Change-Id: I0de058a9eec749a7086138ac2eb79732f06ce55e
Reviewed-on: https://skia-review.googlesource.com/7650
Reviewed-by: Brian Osman <brianosman@google.com>
Commit-Queue: Robert Phillips <robertphillips@google.com>
6 files changed