Add lots of testonly annotations to the GN build.

dbus/BUILD.gn

Index: dbus/BUILD.gn
diff --git a/dbus/BUILD.gn b/dbus/BUILD.gn
index a17f1f13c71a8e53efea89864d89cc189cfd391d..29b38599ff949ce5af55d7bc7722f9e8f3dd2b9b 100644
--- a/dbus/BUILD.gn
+++ b/dbus/BUILD.gn
@@ -55,6 +55,7 @@ proto_library("test_proto") {
 # This target contains mocks that can be used to write unit tests without
 # issuing actual D-Bus calls.
 source_set("test_support") {
+  testonly = true
   sources = [
     "mock_bus.cc",
     "mock_bus.h",
@@ -111,6 +112,7 @@ test("dbus_unittests") {
 }
 
 executable("dbus_test_server") {
+  testonly = true
   sources = [
     "test_server.cc",
     "test_service.cc",