net: make TestDNSThreadLimit execute at the end of tests

Because TestDNSThreadLimit consumes tons of file descriptors and
makes other tests flaky when CGO_ENABLE=0 or being with netgo tag.

Fixes #6580.

R=golang-dev, bradfitz, adg, minux.ma
CC=golang-dev
https://golang.org/cl/14639044

src/pkg/net/dialgoogle_test.go

@@ -107,30 +107,6 @@ var googleaddrsipv4 = []string{
 	"[0:0:0:0:0:ffff::%d.%d.%d.%d]:80",
 }
 
-func TestDNSThreadLimit(t *testing.T) {
-	if testing.Short() || !*testExternal {
-		t.Skip("skipping test to avoid external network")
-	}
-
-	const N = 10000
-	c := make(chan int, N)
-	for i := 0; i < N; i++ {
-		go func(i int) {
-			LookupIP(fmt.Sprintf("%d.net-test.golang.org", i))
-			c <- 1
-		}(i)
-	}
-	// Don't bother waiting for the stragglers; stop at 0.9 N.
-	for i := 0; i < N*9/10; i++ {
-		if i%100 == 0 {
-			//println("TestDNSThreadLimit:", i)
-		}
-		<-c
-	}
-
-	// If we're still here, it worked.
-}
-
 func TestDialGoogleIPv4(t *testing.T) {
 	if testing.Short() || !*testExternal {
 		t.Skip("skipping test to avoid external network")

src/pkg/net/z_last_test.go

@@ -0,0 +1,34 @@
+// Copyright 2009 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 net
+
+import (
+	"fmt"
+	"testing"
+)
+
+func TestDNSThreadLimit(t *testing.T) {
+	if testing.Short() || !*testExternal {
+		t.Skip("skipping test to avoid external network")
+	}
+
+	const N = 10000
+	c := make(chan int, N)
+	for i := 0; i < N; i++ {
+		go func(i int) {
+			LookupIP(fmt.Sprintf("%d.net-test.golang.org", i))
+			c <- 1
+		}(i)
+	}
+	// Don't bother waiting for the stragglers; stop at 0.9 N.
+	for i := 0; i < N*9/10; i++ {
+		if i%100 == 0 {
+			//println("TestDNSThreadLimit:", i)
+		}
+		<-c
+	}
+
+	// If we're still here, it worked.
+}