Add lots of testonly annotations to the GN build.

mojo/common/test/BUILD.gn

Index: mojo/common/test/BUILD.gn
diff --git a/mojo/common/test/BUILD.gn b/mojo/common/test/BUILD.gn
index a0df6d0f5cc6afb3c964f225e807a3a24a970791..b63de6a2c5c9def969731f5a357881896fdef8f8 100644
--- a/mojo/common/test/BUILD.gn
+++ b/mojo/common/test/BUILD.gn
@@ -4,6 +4,7 @@
 
 # GYP version: mojo/mojo_base.gyp:mojo_run_all_unittests
 source_set("run_all_unittests") {
+  testonly = true
   deps = [
     ":test_support_impl",
     "//base",
@@ -18,6 +19,7 @@ source_set("run_all_unittests") {
 
 # GYP version: mojo/mojo_base.gyp:mojo_test_support_impl
 source_set("test_support_impl") {
+  testonly = true
   deps = [
     "//base",
   ]
@@ -30,6 +32,7 @@ source_set("test_support_impl") {
 
 # GYP version: mojo/mojo_base.gyp:mojo_common_test_support
 source_set("test_support") {
+  testonly = true
   deps = [
     "//base",
     "//base/test:test_support",