Add the "testonly" flag to the GN test target.

build/config/BUILDCONFIG.gn

 # Copyright (c) 2013 The Chromium Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
 # =============================================================================
 # BUILD FLAGS
 # =============================================================================
 #
 # This block lists input arguments to the build, along with their default
 # values. GN requires listing them explicitly so it can validate input and have
@@ skipping 552 matching lines @@
       if (defined(invoker.sources)) { sources = invoker.sources }
       if (defined(invoker.visibility)) { visibility = invoker.visibility }
     }
   }
 }
 
 # ==============================================================================
 # TEST SETUP
 # ==============================================================================
 
-# Define a test as an executable. In the future, we'll set "test only" flags
-# on this (when such flags exist) and do something different for Android.
+# Define a test as an executable with the "testonly" flag set. In the future,
+# this will need to be enhanced for Android.
 template("test") {
   executable(target_name) {
     # Configs will always be defined since we set_defaults for a component
     # above. We want to use those rather than whatever came with the nested
     # shared/static library inside the component.
     configs = []  # Prevent list overwriting warning.
     configs = invoker.configs
 
+    testonly = true
+
     # See above call.
     set_sources_assignment_filter([])
 
     if (defined(invoker.all_dependent_configs)) { all_dependent_configs = invoker.all_dependent_configs }
     if (defined(invoker.cflags)) { cflags = invoker.cflags }
     if (defined(invoker.cflags_c)) { cflags_c = invoker.cflags_c }
     if (defined(invoker.cflags_cc)) { cflags_cc = invoker.cflags_cc }
     if (defined(invoker.cflags_objc)) { cflags_objc = invoker.cflags_objc }
     if (defined(invoker.cflags_objcc)) { cflags_objcc = invoker.cflags_objcc }
     if (defined(invoker.data)) { data = invoker.data }
     if (defined(invoker.datadeps)) { datadeps = invoker.datadeps }
     if (defined(invoker.defines)) { defines = invoker.defines }
     if (defined(invoker.deps)) { deps = invoker.deps }
     if (defined(invoker.direct_dependent_configs)) { direct_dependent_configs = invoker.direct_dependent_configs }
     if (defined(invoker.forward_dependent_configs_from)) { forward_dependent_configs_from = invoker.forward_dependent_configs_from }
     if (defined(invoker.include_dirs)) { include_dirs = invoker.include_dirs }
     if (defined(invoker.ldflags)) { ldflags = invoker.ldflags }
     if (defined(invoker.lib_dirs)) { lib_dirs = invoker.lib_dirs }
     if (defined(invoker.libs)) { libs = invoker.libs }
     if (defined(invoker.output_extension)) { output_extension = invoker.output_extension }
     if (defined(invoker.output_name)) { output_name = invoker.output_name }
     if (defined(invoker.public)) { public = invoker.public }
     if (defined(invoker.sources)) { sources = invoker.sources }
     if (defined(invoker.visibility)) { visibility = invoker.visibility }
   }
 }