Use RGBA rather than N32 for GPU tests

This is to support a Mock backend that passes non-rendering context unit tests and has minimal config support.

Change-Id: I57c3ad2d347659d14382a8a7cf53424633bd86c6
Reviewed-on: https://skia-review.googlesource.com/21534
Reviewed-by: Brian Osman <brianosman@google.com>
Commit-Queue: Brian Salomon <bsalomon@google.com>
2 files changed