kselftest/cgroup: add cgroup destruction test

Add new test to verify that a cgroup with dead processes can be destroyed.
The test spawns a child process which allocates and touches 100MB of RAM
to ensure prolonged exit. Subsequently it kills the child, waits until
the cgroup containing the child is empty and destroys the cgroup.

Signed-off-by: Suren Baghdasaryan <surenb@google.com>
[mkoutny@suse.com: Fix typo in test_cgcore_destroy comment]
Acked-by: Michal Koutný <mkoutny@suse.com>
Signed-off-by: Michal Koutný <mkoutny@suse.com>
Signed-off-by: Tejun Heo <tj@kernel.org>