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>
1 file changed