[Bug]: contest memory_policy tests fail on kernels without NUMA

On a kernel built without NUMA, /sys/devices/system/node does not exist and set_mempolicy returns ENOSYS. The memory_policy tests set a policy and expect it to work, so four of them fail: bind_static, default_with_missing_nodes_ok, interleave_without_flags, preferred_relative.

Seen on Ubuntu 26.04 arm64 (OrbStack), kernel 7.0.14:

container stderr was not empty, found : ERROR libcontainer::process::memory_policy: failed to set memory policy err=Nix(ENOSYS)

youki looks right here, it only calls the syscall when the spec asks for one. The group just needs a condition like the tlb tests have, so it skips when the kernel has no NUMA support.

Happy to send the PR.