selftests/bpf: add cgroup_skb netns cookie tests

Add netns cookie test that verifies the helper is now supported and work
in the context of cgroup_skb programs.

Signed-off-by: Mahe Tardy <mahe.tardy@gmail.com>
Link: https://lore.kernel.org/r/20250225125031.258740-2-mahe.tardy@gmail.com


Signed-off-by: Alexei Starovoitov <ast@kernel.org>