GN: add test_app template

This template encapsulates two common features of our test apps:
  - they set testonly, marking them as safe to use testonly libraries
  - they may want instead to be libapp.so on Android.

I plan to follow up to make test_apps push themselves to an Android device when a new argument android_serial is set.

Change-Id: I010d219c14ff6d0cd6a543dd56fd471a67ddb6f1
Reviewed-on: https://skia-review.googlesource.com/6608
Reviewed-by: Derek Sollenberger <djsollen@google.com>
Commit-Queue: Mike Klein <mtklein@chromium.org>
1 file changed