selftests/bpf: Add a new cgroup helper open_classid()

This new helper allows us to obtain the fd of a net_cls cgroup, which will
be utilized in the subsequent patch.

Signed-off-by: Yafang Shao <laoar.shao@gmail.com>
Acked-by: Tejun Heo <tj@kernel.org>
Acked-by: Yonghong Song <yonghong.song@linux.dev>
Link: https://lore.kernel.org/r/20231206115326.4295-3-laoar.shao@gmail.com


Signed-off-by: Martin KaFai Lau <martin.lau@kernel.org>