Add unit tests for for-loop unrolling

Change-Id: I350a6768ac124362b0d3e0f17e7a026265acf804
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/353627
Reviewed-by: Mike Klein <mtklein@google.com>
Commit-Queue: Brian Osman <brianosman@google.com>
6 files changed