selftest/net/ovpn: fix missing file

test-large-mtu.sh is referenced by the Makefile
but does not exist.

Add it along the other scripts.

Fixes: 944f8b6 ("selftest/net/ovpn: extend coverage with more test cases")
Signed-off-by: Antonio Quartulli <antonio@openvpn.net>

Author: ordex

tools/testing/selftests/net/ovpn/test-large-mtu.sh

@@ -0,0 +1,9 @@
+#!/bin/bash
+# SPDX-License-Identifier: GPL-2.0
+# Copyright (C) 2025 OpenVPN, Inc.
+#
+#  Author:	Antonio Quartulli <antonio@openvpn.net>
+
+MTU="1500"
+
+source test.sh