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

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

selftests/bpf: Refactor out some functions in ns_current_pid_tgid test

Refactor some functions in both user space code and bpf program
as these functions are used by later cgroup/sk_msg tests.
Another change is to mark tp program optional loading as later
patches will use optional loading as well since they have quite
different attachment and testing logic.

There is no functionality change.

Signed-off-by: Yonghong Song <yonghong.song@linux.dev>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20240315184904.2976123-1-yonghong.song@linux.dev

bpf, selftests: Fold test_current_pid_tgid_new_ns into test_progs.

Currently tests for bpf_get_ns_current_pid_tgid() are outside test_progs.
This change folds test cases into test_progs.

Changes from v11:

- Fixed test failure is not detected.
- Removed EXIT(3) call as it will stop test_progs execution.

Signed-off-by: Carlos Neira <cneirabustos@gmail.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Link: https://lore.kernel.org/bpf/20210114141033.GA17348@localhost
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

tools/testing/selftests/bpf: Add self-tests for new helper bpf_get_ns_current_pid_tgid.

Self tests added for new helper bpf_get_ns_current_pid_tgid

Signed-off-by: Carlos Neira <cneirabustos@gmail.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/bpf/20200304204157.58695-4-cneirabustos@gmail.com