Move GPU read pixels tests to new file.

To share code with forthcoming write pixels tests.

Bug: skia:8862
Change-Id: I4953cc8b6358ff6514c645296a4baf60b65da905
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/348187
Reviewed-by: Greg Daniel <egdaniel@google.com>
Commit-Queue: Brian Salomon <bsalomon@google.com>
3 files changed