selftests/sched_ext: Test scx_bpf_cgroup_nr_cpus()

Verify that scx_bpf_cgroup_nr_cpus() reports the number of CPUs in a
cgroup's effective cpuset.

Create a cgroup-v2 subtree where a child owns a cpuset and a leaf
inherits it. Sample the kfunc from a BPF_PROG_TYPE_SYSCALL program and
compare it with cpuset.cpus.effective for the root, the cpuset owner and
the inheriting leaf, without a scheduler attached. Attach a scheduler
and verify the value observed from ops.cgroup_init(). Then restrict the
child's cpuset to a non-contiguous and a single-CPU set, and finally
disable cpuset below the parent so that both cgroups inherit the
parent's cpuset.

A disabled cpuset css stays attached to its cgroup until it is
asynchronously offlined, so poll briefly for the last step. Skip the
test when the required controllers or two CPUs are unavailable.

Assisted-by: LLM
Signed-off-by: Andrea Righi <arighi@nvidia.com>
Signed-off-by: Tejun Heo <tj@kernel.org>

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