selftests: cgroup: add values_close_report helper

selftests: cgroup: add values_close_report helper

Some cgroup selftests, such as test_cpu, occasionally fail by a very
small margin and if run in the CI context, it is useful to have detailed
diagnostic output to understand the deviation.

Introduce a values_close_report() helper which performs the same
comparison as values_close(), but prints detailed information when the
values differ beyond the allowed tolerance.

Signed-off-by: Sebastian Chlad <sebastian.chlad@suse.com>
Signed-off-by: Tejun Heo <tj@kernel.org>