Enable intro_flutter_gpu on master CI

Author: domesticmouse

flutter_ci_script_master.sh

@@ -26,8 +26,7 @@ declare -a CODELABS=(
   "haiku_generator"
   "homescreen_codelab"
   "in_app_purchases"
-  # TODO(DomesticMouse): Figure out how to run `flutter test`
-  # "intro_flutter_gpu"
+  "intro_flutter_gpu"
   "namer"
   "next-gen-ui"
   "testing_codelab"