cgroup: Add new test_cpu.c test suite in cgroup selftests

The cgroup selftests suite currently contains tests that validate various
aspects of cgroup, such as validating the expected behavior for memory
controllers, the expected behavior of cgroup.procs, etc. There are no tests
that validate the expected behavior of the cgroup cpu controller.

This patch therefore adds a new test_cpu.c file that will contain cpu
controller testcases. The file currently only contains a single testcase
that validates creating nested cgroups with cgroup.subtree_control
including cpu. Future patches will add more sophisticated testcases that
validate functional aspects of the cpu controller.

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