prober: perform DERP bandwidth probes over TUN device to mimic real client

Updates tailscale/corp#24635

Co-authored-by: Mario Minardi <mario@tailscale.com>
Signed-off-by: Percy Wegmann <percy@tailscale.com>

prober/tun_default.go

@@ -0,0 +1,18 @@
+// Copyright (c) Tailscale Inc & AUTHORS
+// SPDX-License-Identifier: BSD-3-Clause
+
+//go:build !linux && !darwin
+
+package prober
+
+import (
+	"fmt"
+	"net/netip"
+	"runtime"
+)
+
+const tunName = "unused"
+
+func configureTUN(addr netip.Prefix, tunname string) error {
+	return fmt.Errorf("not implemented on " + runtime.GOOS)
+}