t_server_null: add multi-socket testing

t_server_null: add multi-socket testing

This adds a new multi-socket server that listens on IPv4 and IPv6
localhost addresses for TCP and UDP connections respectively. It also
adds two success tests and one failure test with wrong protocol defined
at the client side.

Change-Id: I4ebe1158c36a641888131e824f59004a0f8fb4c5
Signed-off-by: Samuli Seppänen <sasepp@pm.me>
Acked-by: Gert Doering <gert@greenie.muc.de>
Message-Id: <20250708073359.32549-1-gert@greenie.muc.de>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg32061.html
Signed-off-by: Gert Doering <gert@greenie.muc.de>