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

build/config/BUILDCONFIG.gn

Index: build/config/BUILDCONFIG.gn
diff --git a/build/config/BUILDCONFIG.gn b/build/config/BUILDCONFIG.gn
index 6354aec651dd0e4039f12511cb3e2aa8d927b984..c8e8d9ec7d514af8512d7e0e679b268e748cc0c7 100644
--- a/build/config/BUILDCONFIG.gn
+++ b/build/config/BUILDCONFIG.gn
@@ -570,8 +570,8 @@ template("component") {
 # 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
@@ -580,6 +580,8 @@ template("test") {
     configs = []  # Prevent list overwriting warning.
     configs = invoker.configs
 
+    testonly = true
+
     # See above call.
     set_sources_assignment_filter([])