selftests: cgroup: Use values_close_report in test_cpu

Convert test_cpu to use the newly added values_close_report() helper
to print detailed diagnostics when a tolerance check fails. This
provides clearer insight into deviations while run in the CI.

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