bpf: Adjust bpf_tcp_check_syncookie selftest to test dual-stack sockets

The previous commit fixed support for dual-stack sockets in
bpf_tcp_check_syncookie. This commit adjusts the selftest to verify the
fixed functionality.

Signed-off-by: Maxim Mikityanskiy <maximmi@nvidia.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Acked-by: Arthur Fabre <afabre@cloudflare.com>
Link: https://lore.kernel.org/bpf/20220406124113.2795730-2-maximmi@nvidia.com