cgroup: Adding test_cpucg_nested_weight_overprovisioned() testcase

The cgroup cpu controller tests in
tools/testing/selftests/cgroup/test_cpu.c have some testcases that validate
the expected behavior of setting cpu.weight on cgroups, and then hogging
CPUs. What is still missing from the suite is a testcase that validates
nested cgroups. This patch adds test_cpucg_nested_weight_overprovisioned(),
which validates that a parent's cpu.weight will override its children if
they overcommit a host, and properly protect any sibling groups of that
parent.

Signed-off-by: David Vernet <void@manifault.com>
Signed-off-by: Tejun Heo <tj@kernel.org>