selftests: cgroup: skip test_cgcore_lesser_ns_open when cgroup2 mounted without nsdelegate

The test case test_cgcore_lesser_ns_open only tasks effect when cgroup2
is mounted with "nsdelegate" mount option. If it misses this option, or
is remounted without "nsdelegate", the test case will fail. For example,
running bpf/test_cgroup_storage first, and then run cgroup/test_core will
fail on test_cgcore_lesser_ns_open. Skip it if "nsdelegate" is not
detected in cgroup2 mount options.

Fixes: bf35a787 ("selftests: cgroup: Test open-time cgroup namespace usage for migration checks")
Signed-off-by: Tianchen Ding <dtcccc@linux.alibaba.com>
Reviewed-by: Muhammad Usama Anjum <usama.anjum@collabora.com>
Signed-off-by: Tejun Heo <tj@kernel.org>