Start adding unit tests of SkRuntimeEffect

- Change SkRuntimeEffect::Make so *it* can fail, and returns
  [Effect, ErrorText].
- Initial tests just test for expected failure conditions.
  Next steps are to add tests for effects that should work,
  and to validate results on CPU and GPU.

Change-Id: Ibac8c3046104577434034263e9e4a4b177e89129
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/261095
Reviewed-by: Brian Salomon <bsalomon@google.com>
Commit-Queue: Brian Osman <brianosman@google.com>
diff --git a/gn/tests.gni b/gn/tests.gni
index 30c33fe..c7acde0 100644
--- a/gn/tests.gni
+++ b/gn/tests.gni
@@ -242,6 +242,7 @@
   "$_tests/SkRasterPipelineTest.cpp",
   "$_tests/SkRemoteGlyphCacheTest.cpp",
   "$_tests/SkResourceCacheTest.cpp",
+  "$_tests/SkRuntimeEffectTest.cpp",
   "$_tests/SkSLErrorTest.cpp",
   "$_tests/SkSLFPTest.cpp",
   "$_tests/SkSLGLSLTest.cpp",