sync/atomic: remove test dependency on net/http

Depending on net/http means depending on cgo.
When the tree is in a shaky state it's nice to see sync/atomic
pass even if cgo or net causes broken binaries.

R=golang-dev, dave, r
CC=golang-dev
https://golang.org/cl/10753044

src/pkg/sync/example_test.go

@@ -6,10 +6,15 @@ package sync_test
 
 import (
 	"fmt"
-	"net/http"
 	"sync"
 )
 
+type httpPkg struct{}
+
+func (httpPkg) Get(url string) {}
+
+var http httpPkg
+
 // This example fetches several URLs concurrently,
 // using a WaitGroup to block until all the fetches are complete.
 func ExampleWaitGroup() {