tests: Move SkUTF tests to their own file.

Change-Id: Ib4816cae8b4f2d4f11bb03ed043c677a17d89ead
Reviewed-on: https://skia-review.googlesource.com/145147
Reviewed-by: Hal Canary <halcanary@google.com>
Commit-Queue: Hal Canary <halcanary@google.com>
diff --git a/gn/tests.gni b/gn/tests.gni
index b78cc57..7f3b0f5 100644
--- a/gn/tests.gni
+++ b/gn/tests.gni
@@ -236,6 +236,7 @@
   "$_tests/SkSLJITTest.cpp",
   "$_tests/SkSLMemoryLayoutTest.cpp",
   "$_tests/SkSLSPIRVTest.cpp",
+  "$_tests/SkUTFTest.cpp",
   "$_tests/SortTest.cpp",
   "$_tests/SpecialImageTest.cpp",
   "$_tests/SpecialSurfaceTest.cpp",