Add cgroup core selftests

Add cgroup core selftests

This commit adds tests for some of the core functionalities
of cgroups v2.

The commit adds tests for some core principles of croup V2 API:

- test_cgcore_internal_process_constraint

  Tests internal process constraint.
  You can't add a pid to a domain parent if a controller is enabled.

- test_cgcore_top_down_constraint_enable

   Tests that you can't enable a controller on a child if it's not enabled
   on the parent.

- test_cgcore_top_down_constraint_disable

  Tests that you can't disable a controller on a parent if it's
  enabled in a child.

- test_cgcore_no_internal_process_constraint_on_threads

  Tests that there's no internal process constrain on threaded cgroups.
  You can add threads/processes on a parent with a controller enabled.

- test_cgcore_parent_becomes_threaded

  Tests that when a child becomes threaded the parent type becomes
  domain threaded.

- test_cgcore_invalid_domain

  In a situation like:

  A (domain threaded) - B (threaded) - C (domain)

  it tests that C can't be used until it is turned into a threaded cgroup.
  The "cgroup.type" file will report "domain (invalid)" in these cases.
  Operations which fail due to invalid topology use EOPNOTSUPP as the errno.

- test_cgcore_populated

  In a situation like:

  A(0) - B(0) - C(1)
         \ D(0)

  It tests that A, B and C's "populated" fields would be 1 while D's 0.
  It tests that after the one process in C is moved to root, A,B and C's
  "populated" fields would flip to "0" and file modified events will
  be generated on the "cgroup.events" files of both cgroups.

Signed-off-by: Claudio Zumbo <claudioz@fb.com>
Cc: Shuah Khan <shuah@kernel.org>
Cc: Roman Gushchin <guro@fb.com>
Cc: Tejun Heo <tj@kernel.org>
Cc: kernel-team@fb.com
Acked-by: Tejun Heo <tj@kernel.org>
Reviewed-by: Roman Gushchin <guro@fb.com>
Signed-off-by: Shuah Khan (Samsung OSG) <shuah@kernel.org>