ftests: add a test for cgroup v2 threaded support

ftests: add a test for cgroup v2 threaded support

Add a functional test to verify threaded cgroup support on a
cgroup v2 filesystem.

-----------------------------------------------------------------
Test Results:
        Run Date:                          Jan 25 10:41:34
        Passed:                                  1 test(s)
        Skipped:                                 0 test(s)
        Failed:                                  0 test(s)
-----------------------------------------------------------------
Timing Results:
        Test                          Time (sec)
        ----------------------------------------
        setup                               0.00
        036-cgset-multi_thread.py           0.10
        teardown                            0.00
        ----------------------------------------
        Total Run Time                      0.10

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>
TJH: Minor cleanups to utilize the new set_and_validate() method