net: don't leak test helper goroutine in TestAcceptTimeout

Fixes #15109.

Change-Id: Ibfdedd6807322ebec84bacfeb492fb53fe066960
Reviewed-on: https://go-review.googlesource.com/23742
Run-TryBot: Mikio Hara <mikioh.mikioh@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Marcel van Lohuizen <mpvl@golang.org>

src/net/timeout_test.go

@@ -5,6 +5,7 @@
 package net
 
 import (
+	"context"
 	"fmt"
 	"internal/testenv"
 	"io"
@@ -164,10 +165,13 @@ func TestAcceptTimeout(t *testing.T) {
 	}
 	defer ln.Close()
 
+	ctx, cancel := context.WithCancel(context.Background())
+	defer cancel()
 	for i, tt := range acceptTimeoutTests {
 		if tt.timeout < 0 {
 			go func() {
-				c, err := Dial(ln.Addr().Network(), ln.Addr().String())
+				var d Dialer
+				c, err := d.DialContext(ctx, ln.Addr().Network(), ln.Addr().String())
 				if err != nil {
 					t.Error(err)
 					return