selftests: ovpn: add test for the FW mark feature

selftests: ovpn: add test for the FW mark feature

Add a selftest to verify that the FW mark socket option is correctly
supported and its value propagated by ovpn.

The test adds and removes nftables DROP rules based on the mark value,
and checks that the rule counter aligns with the number of lost ping
packets.

Cc: Shuah Khan <shuah@kernel.org>
Cc: linux-kselftest@vger.kernel.org
Cc: horms@kernel.org
Signed-off-by: Ralf Lici <ralf@mandelbit.com>
Signed-off-by: Antonio Quartulli <antonio@openvpn.net>