cgroup: selftests: Add tests for freezer time

Test cgroup v2 freezer time stat. Freezer time accounting should
be independent of other cgroups in the hierarchy and should increase
iff a cgroup is CGRP_FREEZE (regardless of whether it reaches
CGRP_FROZEN).

Skip these tests on systems without freeze time accounting.

Signed-off-by: Tiffany Yang <ynaffit@google.com>
Cc: Michal Koutný <mkoutny@suse.com>
Signed-off-by: Tejun Heo <tj@kernel.org>