make test consistent

Don't pass rand() to a function as a pair of
parameters, since the order of evaluation
is unknown

TBR=robertphillips@google.com
GOLD_TRYBOT_URL= https://gold.skia.org/search2?unt=true&query=source_type%3Dgm&master=false&issue=1682203002

Review URL: https://codereview.chromium.org/1682203002
1 file changed