cgroup: Add test_cpucg_nested_weight_underprovisioned() testcase

The cgroup cpu controller test suite currently contains a testcase called
test_cpucg_nested_weight_underprovisioned() which verifies the expected
behavior of cpu.weight when applied to nested cgroups. That first testcase
validated the expected behavior when the processes in the leaf cgroups
overcommitted the system. This patch adds a complementary
test_cpucg_nested_weight_underprovisioned() testcase which validates
behavior when those leaf cgroups undercommit the system.

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