various: disable MPTCP when setting TCP_USER_TIMEOUT sockopt (#15063)

There's nothing about it on
https://github.com/multipath-tcp/mptcp_net-next/issues/ but empirically
MPTCP doesn't support this option on awly's kernel 6.13.2 and in GitHub
actions.

Updates #15015

Signed-off-by: Andrew Lytvynov <awly@tailscale.com>

net/ktimeout/ktimeout_linux_test.go

@@ -4,17 +4,22 @@
 package ktimeout
 
 import (
+	"context"
 	"net"
 	"testing"
 	"time"
 
-	"golang.org/x/net/nettest"
 	"golang.org/x/sys/unix"
 	"tailscale.com/util/must"
 )
 
 func TestSetUserTimeout(t *testing.T) {
-	l := must.Get(nettest.NewLocalListener("tcp"))
+	lc := net.ListenConfig{}
+	// As of 2025-02-19, MPTCP does not support TCP_USER_TIMEOUT socket option
+	// set in ktimeout.UserTimeout above.
+	lc.SetMultipathTCP(false)
+
+	l := must.Get(lc.Listen(context.Background(), "tcp", "localhost:0"))
 	defer l.Close()
 
 	var err error