selftests/cgroup: add tests for cloning into cgroups

Expand the cgroup test-suite to include tests for CLONE_INTO_CGROUP.
This adds the following tests:
- CLONE_INTO_CGROUP manages to clone a process directly into a correctly
  delegated cgroup
- CLONE_INTO_CGROUP fails to clone a process into a cgroup that has been
  removed after we've opened an fd to it
- CLONE_INTO_CGROUP fails to clone a process into an invalid domain
  cgroup
- CLONE_INTO_CGROUP adheres to the no internal process constraint
- CLONE_INTO_CGROUP works with the freezer feature

Cc: Roman Gushchin <guro@fb.com>
Cc: Tejun Heo <tj@kernel.org>
Cc: Shuah Khan <shuah@kernel.org>
Cc: cgroups@vger.kernel.org
Cc: linux-kselftest@vger.kernel.org
Signed-off-by: Christian Brauner <christian.brauner@ubuntu.com>
---
/* v1 */
Link: https://lore.kernel.org/r/20191218173516.7875-4-christian.brauner@ubuntu.com

/* v2 */
Link: https://lore.kernel.org/r/20191223061504.28716-4-christian.brauner@ubuntu.com
unchanged

/* v3 */
Link: https://lore.kernel.org/r/20200117002143.15559-6-christian.brauner@ubuntu.com
unchanged

/* v3 */
unchanged
5 files changed