selftests/bpf: Add a cgroup prog bpf_get_ns_current_pid_tgid() test

Add a cgroup bpf program test where the bpf program is running
in a pid namespace. The test is successfully:
  #165/3   ns_current_pid_tgid/new_ns_cgrp:OK

Signed-off-by: Yonghong Song <yonghong.song@linux.dev>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20240315184910.2976522-1-yonghong.song@linux.dev