cgroup: Add test_cpucg_weight_underprovisioned() testcase

test_cpu.c includes testcases that validate the cgroup cpu controller.
This patch adds a new testcase called test_cpucg_weight_underprovisioned()
that verifies that processes with different cpu.weight that are all running
on an underprovisioned system, still get roughly the same amount of cpu
time.

Because test_cpucg_weight_underprovisioned() is very similar to
test_cpucg_weight_overprovisioned(), this patch also pulls the common logic
into a separate helper function that is invoked from both testcases, and
which uses function pointers to invoke the unique portions of the
testcases.

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