Add lots of testonly annotations to the GN build.

ash/BUILD.gn

Index: ash/BUILD.gn
diff --git a/ash/BUILD.gn b/ash/BUILD.gn
index a52394bc2974e978278803a5ff7bbbde9b723827..fb546dc5d95ddee44ff8c077c6059279e74ee844 100644
--- a/ash/BUILD.gn
+++ b/ash/BUILD.gn
@@ -165,6 +165,7 @@ component("ash_with_content") {
 }
 
 static_library("test_support") {
+  testonly = true
   sources = gypi_values.ash_test_support_sources
 
   deps = [
@@ -198,6 +199,7 @@ static_library("test_support") {
 }
 
 static_library("ash_shell_lib") {
+  testonly = true
   sources = gypi_values.ash_shell_lib_sources
 
   deps = [
@@ -338,6 +340,7 @@ test("ash_unittests") {
 }
 
 executable("ash_shell") {
+  testonly = true
   sources = [
     "shell/shell_main.cc",
   ]