Fix Unpremul Index8 tests

TBR=scroggo@google.com

BUG=skia:
GOLD_TRYBOT_URL= https://gold.skia.org/search2?unt=true&query=source_type%3Dgm&master=false&issue=1663323002

Review URL: https://codereview.chromium.org/1663323002
1 file changed