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