add flatten test to SkFont

Change-Id: I7e96e826fad9aa204c963ae671cce668a676378f
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/253684
Reviewed-by: Florin Malita <fmalita@chromium.org>
Commit-Queue: Mike Reed <reed@google.com>
diff --git a/gn/tests.gni b/gn/tests.gni
index 608e8c9..637304f 100644
--- a/gn/tests.gni
+++ b/gn/tests.gni
@@ -86,6 +86,7 @@
   "$_tests/FontMgrFontConfigTest.cpp",
   "$_tests/FontMgrTest.cpp",
   "$_tests/FontNamesTest.cpp",
+  "$_tests/FontTest.cpp",
   "$_tests/FrontBufferedStreamTest.cpp",
   "$_tests/GeometryTest.cpp",
   "$_tests/GifTest.cpp",