cmd/protoc-gen-go/testdata: add registry test

This test verifies that the inialization logic does not accidentally
execute the lazy initialization logic that is intended to only evaluate
upon first use.

Change-Id: I5e9dea17ce88081c78195af0e86dd38223689f63
Reviewed-on: https://go-review.googlesource.com/c/protobuf/+/184259
Reviewed-by: Damien Neil <dneil@google.com>

cmd/protoc-gen-go/testdata/registry_test.go

@@ -0,0 +1,27 @@
+// Copyright 2019 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style.
+// license that can be found in the LICENSE file.
+
+package main
+
+import (
+	"testing"
+
+	"google.golang.org/protobuf/internal/filedesc"
+	"google.golang.org/protobuf/reflect/protoreflect"
+	"google.golang.org/protobuf/reflect/protoregistry"
+)
+
+func TestRegistry(t *testing.T) {
+	var hasFiles bool
+	protoregistry.GlobalFiles.RangeFiles(func(fd protoreflect.FileDescriptor) bool {
+		if fd.(*filedesc.File).L2 != nil {
+			t.Errorf("file %q eagerly went through lazy initialization", fd.Path())
+		}
+		hasFiles = true
+		return true
+	})
+	if !hasFiles {
+		t.Errorf("protoregistry.GlobalFiles is empty")
+	}
+}