Make testing/ pass gn check.

Change-Id: Ifb2d60bf3c2d86a105705196d65e318081cefe4e
Reviewed-on: https://pdfium-review.googlesource.com/c/43981
Reviewed-by: Tom Sepez <tsepez@chromium.org>
Commit-Queue: Lei Zhang <thestig@chromium.org>
diff --git a/.gn b/.gn
index 2c77331..e9335c5 100644
--- a/.gn
+++ b/.gn
@@ -20,12 +20,13 @@
   "//core/fxcrt/*",
   "//fxbarcode/*",
   "//samples/*",
+  "//testing/:*",
+  "//testing/fuzzers/*",
+  "//testing/image_diff/*",
 
   # TODO(thestig): Work on these.
   #"//core/*",
   #"//fpdfsdk/*",
   #"//fxjs/*",
-  #"//testing/*",
-  #"//testing/fuzzers/*",
   #"//xfa/*",
 ]