add test for custom hardmix blendmode

Change-Id: Idf2eca6af4d82a03d0fc0d788bfffcb15f46bd29
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/566340
Auto-Submit: Jorge Betancourt <jmbetancourt@google.com>
Commit-Queue: Jorge Betancourt <jmbetancourt@google.com>
Reviewed-by: Florin Malita <fmalita@google.com>
Commit-Queue: Florin Malita <fmalita@google.com>
1 file changed