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>
diff --git a/gn/tests.gni b/gn/tests.gni
index 4350ef2..be368f0 100644
--- a/gn/tests.gni
+++ b/gn/tests.gni
@@ -141,6 +141,7 @@
   "$_tests/PaintBreakTextTest.cpp",
   "$_tests/PaintImageFilterTest.cpp",
   "$_tests/PaintTest.cpp",
+  "$_tests/ParametricStageTest.cpp",
   "$_tests/ParsePathTest.cpp",
   "$_tests/PathCoverageTest.cpp",
   "$_tests/PathMeasureTest.cpp",