tests/cgroup: Add None as controller get_controller_mount_point()

tests/cgroup: Add None as controller get_controller_mount_point()

Cgroup hybrid/unified setup modes support creating cgroups without
attaching them to any controller.  Enhance get_controller_mount_point()
to support 'None' as a valid controller and return a mount point.

Hybrid Setup
------------
>>> from cgroup import Cgroup
>>> Cgroup.get_controller_mount_point(None)
'/sys/fs/cgroup/unified'

Unified Setup
-------------
>>> from cgroup import Cgroup
>>> Cgroup.get_controller_mount_point(None)
'/sys/fs/cgroup'

Suggested-by: Tom Hromatka <tom.hromatka@oracle.com>
Signed-off-by: Kamalesh Babulal <kamalesh.babulal@oracle.com>
Signed-off-by: Tom Hromatka <tom.hromatka@oracle.com>