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

Add a sk_msg bpf program test where the program is running in a pid
namespace. The test is successful:
  #165/4   ns_current_pid_tgid/new_ns_sk_msg:OK

Signed-off-by: Yonghong Song <yonghong.song@linux.dev>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20240315184915.2976718-1-yonghong.song@linux.dev