selftests/bpf: add get_next_key boundary test for cgroup_storage

selftests/bpf: add get_next_key boundary test for cgroup_storage

Verify that bpf_map__get_next_key() correctly returns -ENOENT when
called on the last (and only) key in a cgroup_storage map. Before the
fix in the previous patch, this would succeed with bogus key data
instead of failing.

Suggested-by: Paul Chaignon <paul.chaignon@gmail.com>
Signed-off-by: Weiming Shi <bestswngs@gmail.com>
Acked-by: Paul Chaignon <paul.chaignon@gmail.com>
Link: https://lore.kernel.org/r/20260403132951.43533-3-bestswngs@gmail.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>