selftests/bpf: Add test for using css_task iter in sleepable progs

This Patch add a test to prove css_task iter can be used in normal
sleepable progs.

Signed-off-by: Chuyi Zhou <zhouchuyi@bytedance.com>
Acked-by: Yonghong Song <yonghong.song@linux.dev>
Link: https://lore.kernel.org/r/20231031050438.93297-4-zhouchuyi@bytedance.com


Signed-off-by: Alexei Starovoitov <ast@kernel.org>