[graphite] Add unit test for new Graphite-specific makeSubset API

Bug: b/237111152
Change-Id: Iba9a9ce855c349dda1fdf20d756c0c92afba5347
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/629920
Commit-Queue: Robert Phillips <robertphillips@google.com>
Reviewed-by: James Godfrey-Kittle <jamesgk@google.com>
1 file changed