Switch BackendAllocationTests from GrPixelConfig to GrColorType

Bug: skia:6718
Change-Id: Ifd78a1b227dc468ce882cf3caf78b4fae80cc006
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/229386
Commit-Queue: Robert Phillips <robertphillips@google.com>
Reviewed-by: Greg Daniel <egdaniel@google.com>
Reviewed-by: Jim Van Verth <jvanverth@google.com>
6 files changed