use the cgroup.controllers file instead of cgroup.subtree_control to
read the list of controllers available in the current cgroup.
Closes: https://github.com/containers/podman/issues/11931
[NO TESTS NEEDED] we have disabled this test in the CI because it is
difficult to know what controllers are going to be enabled for
rootless under all conditions we test.
Signed-off-by: Giuseppe Scrivano <gscrivan@redhat.com>