GN: add test_app template
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 <d9b232705cad36bae5cfccf14f9650bb292e6c71@google.com>
Commit-Queue: Mike Klein <14574f09dfa9b4e14759b88c3426a495a0e627b0@chromium.org>

google/skia,aosp-mirror/platform_external_skia,HalCanary/skia-hc,HalCanary/skia-hc,google/skia,rubenvb/skia,aosp-mirror/platform_external_skia,Hikari-no-Tenshi/android_external_skia,HalCanary/skia-hc,rubenvb/skia,Hikari-no-Tenshi/android_external_skia,HalCanary/skia-hc,Hikari-no-Tenshi/android_external_skia,Hikari-no-Tenshi/android_external_skia,aosp-mirror/platform_external_skia,google/skia,aosp-mirror/platform_external_skia,aosp-mirror/platform_external_skia,Hikari-no-Tenshi/android_external_skia,rubenvb/skia,google/skia,google/skia,aosp-mirror/platform_external_skia,rubenvb/skia,HalCanary/skia-hc,HalCanary/skia-hc,rubenvb/skia,rubenvb/skia,google/skia,Hikari-no-Tenshi/android_external_skia,rubenvb/skia,google/skia,aosp-mirror/platform_external_skia,HalCanary/skia-hc,aosp-mirror/platform_external_skia,HalCanary/skia-hc,rubenvb/skia,google/skia,google/skia,HalCanary/skia-hc,rubenvb/skia,HalCanary/skia-hc,aosp-mirror/platform_external_skia,Hikari-no-Tenshi/android_external_skia,Hikari-no-Tenshi/android_external_skia,aosp-mirror/platform_external_skia,rubenvb/skia,google/skia