selftests/bpf: Add tests for css_task iter combining with cgroup iter

selftests/bpf: Add tests for css_task iter combining with cgroup iter

This patch adds a test which demonstrates how css_task iter can be combined
with cgroup iter and it won't cause deadlock, though cgroup iter is not
sleepable.

Signed-off-by: Chuyi Zhou <[email protected]>
Acked-by: Yonghong Song <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Alexei Starovoitov <[email protected]>