net: reduce dial tests on netbsd

Add NetBSD to the list of operating systems that have a reduced set
of dial tests.

R=golang-dev, r
CC=golang-dev
https://golang.org/cl/6208083

src/pkg/net/dial_test.go

@@ -130,7 +130,7 @@ func TestSelfConnect(t *testing.T) {
 		n = 1000
 	}
 	switch runtime.GOOS {
-	case "darwin", "freebsd", "openbsd", "windows":
+	case "darwin", "freebsd", "netbsd", "openbsd", "windows":
 		// Non-Linux systems take a long time to figure
 		// out that there is nothing listening on localhost.
 		n = 100