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

mojo/BUILD.gn

Index: mojo/BUILD.gn
diff --git a/mojo/BUILD.gn b/mojo/BUILD.gn
index 2be3382754f4f9df7caf8a1a8946c3ff202f67b8..61e812316b480151ba8dcdf29dd981843e3f8980 100644
--- a/mojo/BUILD.gn
+++ b/mojo/BUILD.gn
@@ -5,6 +5,8 @@
 import("//build/config/ui.gni")
 
 group("mojo") {
+  # Meta-target, don't link into production code.
+  testonly = true
   deps = [
     ":tests",
     "//mojo/common",
@@ -22,6 +24,7 @@ group("mojo") {
 }
 
 group("tests") {
+  testonly = true
   deps = [
     "//mojo/application_manager:mojo_application_manager_unittests",
     "//mojo/apps/js/test:mojo_apps_js_unittests",