add unit tests for parametric_* stages

Change-Id: I7fab6d1c7240c17f2cc8436e8c6e7c8d2df940bb
Reviewed-on: https://skia-review.googlesource.com/13814
Reviewed-by: Matt Sarett <msarett@google.com>
Commit-Queue: Mike Klein <mtklein@chromium.org>
2 files changed