selftest/net/ovpn: fix TCP socket creation

TCP sockets cannot be created with AF_UNSPEC, but
one among the supported family must be used.

Since commit 944f8b6a ("selftest/net/ovpn: extend
coverage with more test cases") the default address
family for all tests was changed from AF_INET to AF_UNSPEC,
thus breaking all TCP cases.

Restore AF_INET as default address family for TCP listeners.

Fixes: 944f8b6a ("selftest/net/ovpn: extend coverage with more test cases")
Signed-off-by: Antonio Quartulli <antonio@openvpn.net>