Add helper to create random AsFPArgs for Ganesh unit tests

Add helper to create random AsFPArgs for Ganesh unit tests

Reduces copy-paste and eases maintenance. I'll be adding another field to
AsFPArgs soon, and this is going to streamline that change.

BUG=skia:

GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=3639

Change-Id: I6372ed5dce50a5ba9d73039bd4714e34502a1f75
Reviewed-on: https://skia-review.googlesource.com/3639
Commit-Queue: Brian Osman <brianosman@google.com>
Reviewed-by: Brian Salomon <bsalomon@google.com>