Move GPU read pixels tests to new file.

To share code with forthcoming write pixels tests.

Bug: skia:8862
Change-Id: I4953cc8b6358ff6514c645296a4baf60b65da905
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/348187
Reviewed-by: Greg Daniel <egdaniel@google.com>
Commit-Queue: Brian Salomon <bsalomon@google.com>
diff --git a/gn/tests.gni b/gn/tests.gni
index 6b65692..318e223 100644
--- a/gn/tests.gni
+++ b/gn/tests.gni
@@ -215,6 +215,7 @@
   "$_tests/RTreeTest.cpp",
   "$_tests/RandomTest.cpp",
   "$_tests/ReadPixelsTest.cpp",
+  "$_tests/ReadWritePixelsGpuTest.cpp",
   "$_tests/RecordDrawTest.cpp",
   "$_tests/RecordOptsTest.cpp",
   "$_tests/RecordPatternTest.cpp",