[graphite] Test larger transfers in TextureFormatTest

Follow-up CLs had bugs in their SIMD implementations that were only
present in lanes after the 1st pixel, so weren't caught by the
existing 1x1 transfer test.  This adds an additional larger transfer
that assumes constant input and confirms all outputs are the same.

Bug: 509890794
Change-Id: I0cafce10dbe4287ce0a10c100e8f4f6c271d75f6
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/1308956
Commit-Queue: Michael Ludwig <michaelludwig@google.com>
Reviewed-by: Robert Phillips <robertphillips@google.com>