ftests: Rework test 071 to use cgroup_write_systemd_default_cgroup()

ftests: Rework test 071 to use cgroup_write_systemd_default_cgroup()

cgroup_write_systemd_default_cgroup() populates libcgroup's /var/run
file with the default slice/scope.  No verification is done at the time
of writing.

cgroup_set_default_systemd_cgroup() reads the /var/run file and verifies
that it points to a valid path.  If so, it populates libcgroup's global
variable, systemd_default_cgroup.  systemd_default_cgroup is then used
in any subsequent building of libcgroup paths.

Previously, test 071 tested cgroup_set_default_systemd_cgroup() but it
didn't exercise cgroup_write_systemd_default_cgroup().  Rework test 071
to exercise the most likely combinations of writing/setting the default
path that a user would encounter.

-----------------------------------------------------------------
Test Results:
	Run Date:                          May 24 07:32:54
	Passed:                                  1 test(s)
	Skipped:                                 0 test(s)
	Failed:                                  0 test(s)
-----------------------------------------------------------------
Timing Results:
	Test                                       Time (sec)
	-----------------------------------------------------
	setup                                            0.00
	071-sudo-set_default_systemd_cgroup.py           4.67
	teardown                                         0.00
	-----------------------------------------------------
	Total Run Time                                   4.67

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