net: use DialTimeout in TestSelfConnect

This avoids problems with systems that take a long time to
find out nothing is listening, while still testing for the
self-connect misfeature since a self-connect should be fast.
With this we may be able to remove the test for non-Linux
systems.

Tested (on GNU/Linux) by editing selfConnect in
tcpsock_posix.go to always return false and verifying that
TestSelfConnect then fails with and without this change.

Idea from Uros Bizjak.

R=golang-codereviews, bradfitz
CC=golang-codereviews
https://golang.org/cl/39200044

src/pkg/net/dial_test.go

@@ -147,7 +147,7 @@ func TestSelfConnect(t *testing.T) {
 		n = 100
 	}
 	for i := 0; i < n; i++ {
-		c, err := Dial("tcp", addr)
+		c, err := DialTimeout("tcp", addr, time.Millisecond)
 		if err == nil {
 			c.Close()
 			t.Errorf("#%d: Dial %q succeeded", i, addr)