Consolidate read/write-Pixels testing code - take 2

Change-Id: I0de058a9eec749a7086138ac2eb79732f06ce55e
Reviewed-on: https://skia-review.googlesource.com/7650
Reviewed-by: Brian Osman <brianosman@google.com>
Commit-Queue: Robert Phillips <robertphillips@google.com>
diff --git a/gn/tests.gni b/gn/tests.gni
index 483f574..6a836f6 100644
--- a/gn/tests.gni
+++ b/gn/tests.gni
@@ -234,6 +234,8 @@
   "$_tests/Test.cpp",
   "$_tests/TestConfigParsing.cpp",
   "$_tests/TestTest.cpp",
+  "$_tests/TestUtils.h",
+  "$_tests/TestUtils.cpp",
   "$_tests/TextBlobCacheTest.cpp",
   "$_tests/TextBlobTest.cpp",
   "$_tests/TextureCompressionTest.cpp",