net: make TestSelfConnect less fragile

We believe TestSelfConnect can accidentally connect to
something else listening on or dialing from that port.

Fixes #8680.

LGTM=bradfitz
R=bradfitz
CC=golang-codereviews, rlh
https://golang.org/cl/136700043

src/net/dial_test.go

@@ -119,6 +119,7 @@ func TestSelfConnect(t *testing.T) {
 		// TODO(brainman): do not know why it hangs.
 		t.Skip("skipping known-broken test on windows")
 	}
+
 	// Test that Dial does not honor self-connects.
 	// See the comment in DialTCP.
 
@@ -149,8 +150,12 @@ func TestSelfConnect(t *testing.T) {
 	for i := 0; i < n; i++ {
 		c, err := DialTimeout("tcp", addr, time.Millisecond)
 		if err == nil {
+			if c.LocalAddr().String() == addr {
+				t.Errorf("#%d: Dial %q self-connect", i, addr)
+			} else {
+				t.Logf("#%d: Dial %q succeeded - possibly racing with other listener", i, addr)
+			}
 			c.Close()
-			t.Errorf("#%d: Dial %q succeeded", i, addr)
 		}
 	}
 }