Supply random dst color space to asFP in unit tests

Supply random dst color space to asFP in unit tests

TBR=bsalomon@google.com (Testing-only API change)

BUG=skia:

GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=2876

Change-Id: I0ca26da0307848cdfc8ffaac2d042601663ab00b
Reviewed-on: https://skia-review.googlesource.com/2876
Reviewed-by: Brian Salomon <bsalomon@google.com>
Reviewed-by: Mike Reed <reed@google.com>
Reviewed-by: Robert Phillips <robertphillips@google.com>
Commit-Queue: Brian Osman <brianosman@google.com>