Avoid using GrGpu::readPixels in GrTestingBackendTextureUploadTest.

Change-Id: Ic7ba918177f6f5cc35da713cf063c467bd7a7908
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/220999
Reviewed-by: Greg Daniel <egdaniel@google.com>
Commit-Queue: Brian Salomon <bsalomon@google.com>
2 files changed