selftests/bpf: Verify bpf_get_netns_cookie in BPF_PROG_TYPE_CGROUP_SOCKOPT

Add extra calls to sockopt_sk.c.

Signed-off-by: Stanislav Fomichev <sdf@google.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Acked-by: Martin KaFai Lau <kafai@fb.com>
Link: https://lore.kernel.org/bpf/20210813230530.333779-3-sdf@google.com