Split out check_base_readbacks sub-test in BackendAllocation tests

We want to reuse this sub-portion of the tests for SkPixmap initialized backend textures

Note: this also changes the format read-back into to be RGBA8

Change-Id: I3708541cb4e60cf2ba7b587da6e21169ef890b04
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/244056
Reviewed-by: Greg Daniel <egdaniel@google.com>
Commit-Queue: Robert Phillips <robertphillips@google.com>
1 file changed