selftests: cgroup: Optionally set up v1 environment

Use the missing mount of the unifier hierarchy as a hint of legacy
system and prepare our own named v1 hierarchy for tests.

The code is only in test_core.c and not cgroup_util.c because other
selftests are related to controllers which will be exposed on v2
hierarchy but named hierarchies are only v1 thing.

Signed-off-by: Michal Koutný <mkoutny@suse.com>
Signed-off-by: Tejun Heo <tj@kernel.org>