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

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

[ Upstream commit 87ade6cd859ea9dbde6e80b3fcf717ed9a73b4a9 ]

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
Stable-dep-of: 21f0b0af9772 ("selftests/bpf: Fix include of <sys/fcntl.h>")
Signed-off-by: Sasha Levin <sashal@kernel.org>