net: add test cases for the both of netgo, cgo DNS lookups

Update #4078

R=golang-dev, rsc
CC=golang-dev
https://golang.org/cl/14638043

src/pkg/net/cgo_unix_test.go

+// Copyright 2013 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.
+
+// +build cgo,!netgo
+// +build darwin dragonfly freebsd linux netbsd openbsd
+
+package net
+
+import "testing"
+
+func TestCgoLookupIP(t *testing.T) {
+	host := "localhost"
+	_, err, ok := cgoLookupIP(host)
+	if !ok {
+		t.Errorf("cgoLookupIP must not be a placeholder")
+	}
+	if err != nil {
+		t.Errorf("cgoLookupIP failed: %v", err)
+	}
+	if _, err := goLookupIP(host); err != nil {
+		t.Errorf("goLookupIP failed: %v", err)
+	}
+}

src/pkg/net/netgo_unix_test.go

+// Copyright 2013 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.
+
+// +build !cgo netgo
+// +build darwin dragonfly freebsd linux netbsd openbsd
+
+package net
+
+import "testing"
+
+func TestGoLookupIP(t *testing.T) {
+	host := "localhost"
+	_, err, ok := cgoLookupIP(host)
+	if ok {
+		t.Errorf("cgoLookupIP must be a placeholder")
+	}
+	if err != nil {
+		t.Errorf("cgoLookupIP failed: %v", err)
+	}
+	if _, err := goLookupIP(host); err != nil {
+		t.Errorf("goLookupIP failed: %v", err)
+	}
+}