net: make TCPStress test shorter

It timeouts on freebsd builders:
http://build.golang.org/log/3d8169e13bff912bebf6fd3c54b34ad2d29a7221
but there are always runnable goroutines,
which suggests that it's slowly progressing.

R=golang-dev, bradfitz
CC=golang-dev
https://golang.org/cl/12939043

src/pkg/net/tcp_test.go

@@ -497,8 +497,11 @@ func TestTCPReadWriteMallocs(t *testing.T) {
 
 func TestTCPStress(t *testing.T) {
 	const conns = 2
-	const msgs = 1e4
 	const msgLen = 512
+	msgs := int(1e4)
+	if testing.Short() {
+		msgs = 1e2
+	}
 
 	sendMsg := func(c Conn, buf []byte) bool {
 		n, err := c.Write(buf)