Set gn output_dir variable for test apps.

Fixes location of executable for xcode project generation.

Change-Id: I256c4a8f5e784f290f872094ada9949f23e6050b
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/281578
Reviewed-by: Mike Reed <reed@google.com>
Commit-Queue: Jim Van Verth <jvanverth@google.com>

BUILD.gn

@@ -1269,6 +1269,7 @@ if (skia_enable_tools) {
     } else {
       # !is_ios
 
+      output_dir = root_build_dir
       if (defined(invoker.is_shared_library) && invoker.is_shared_library) {
         shared_library("lib" + target_name) {
           forward_variables_from(invoker, "*", [ "is_shared_library" ])