gunit/test-012: Create subtree_control for leaf cgroup

gunit/test-012: Create subtree_control for leaf cgroup

The test case relies on the parent cgroup's subtree_control file, while
creating a cgroup to verify the controllers enabled in created cgroup v2
and the current code cleverly emulates it.  With the change to enable
the controller at the leaf cgroup this emulation gets difficult, so
introduce a dummy cgroup v2 mount that will help in the construction of
the path when the cgroup v2 is created with the empty controller because
the brilliance lies in emulating it by manually creating the
subtree_control file to give the impression of enabling the controller
in the cgroup v2 cgroup.

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