selftests/bpf: Add selftests for bpf_cgroup_ancestor() kfunc

bpf_cgroup_ancestor() allows BPF programs to access the ancestor of a
struct cgroup *. This patch adds selftests that validate its expected
behavior.

Signed-off-by: David Vernet <void@manifault.com>
Link: https://lore.kernel.org/r/20221122055458.173143-5-void@manifault.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

tools/testing/selftests/bpf/prog_tests/cgrp_kfunc.c

@@ -86,6 +86,7 @@ static const char * const success_tests[] = {
 	"test_cgrp_acquire_leave_in_map",
 	"test_cgrp_xchg_release",
 	"test_cgrp_get_release",
+	"test_cgrp_get_ancestors",
 };
 
 static struct {