x/crypto/ssh/test: test all key exchanges against sshd.

Change-Id: I9d775d6e9c39bee1f2bf914ba3659acb6b0029dd
Reviewed-on: https://go-review.googlesource.com/13591
Reviewed-by: Adam Langley <agl@golang.org>

ssh/test/session_test.go

@@ -319,3 +319,22 @@ func TestMACs(t *testing.T) {
 		}
 	}
 }
+
+func TestKeyExchanges(t *testing.T) {
+	var config ssh.Config
+	config.SetDefaults()
+	kexOrder := config.KeyExchanges
+	for _, kex := range kexOrder {
+		server := newServer(t)
+		defer server.Shutdown()
+		conf := clientConfig()
+		// Don't fail if sshd doesnt have the kex.
+		conf.KeyExchanges = append([]string{kex}, kexOrder...)
+		conn, err := server.TryDial(conf)
+		if err == nil {
+			conn.Close()
+		} else {
+			t.Errorf("failed for kex %q", kex)
+		}
+	}
+}