[build] Work around test_spec() missing testonly in go_test()

test_spec() should set testonly=true automatically, but that is
blocked on cleaning up the Rust templates.  For now, fix the
problem for go_test() uses of test_spec() by setting it directly.

Bug: BLD-493 #comment Work around test_spec() missing testonly in go_test()
Change-Id: Ibaa385be3ddeb01540f7ef7426187e6836cec6f2
diff --git a/build/go/go_test.gni b/build/go/go_test.gni
index 01eb6c7..08034a7 100644
--- a/build/go/go_test.gni
+++ b/build/go/go_test.gni
@@ -31,11 +31,14 @@
   _output_path = "${root_out_dir}/${_output_name}"
 
   _is_disabled = defined(invoker.disabled) && invoker.disabled
+
   # We only include test metadata for non-disabled, linux/mac tests; metadata
   # for fuchsia tests is aggregated in the package template.
   _include_test_metadata = !_is_disabled && (is_linux || is_mac)
   if (_include_test_metadata) {
     test_spec(_test_spec_target_name) {
+      # TODO(BLD-493): testonly should be automatic here
+      testonly = true
       name = _main_target_name
       location = _output_path
       deps = []