ftests: add test to replace scope idle_thread using cgexec

ftests: add test to replace scope idle_thread using cgexec

Add a test to replace systemd scope default idle_thread using cgexec
'-r' flag.

-----------------------------------------------------------------
Test Results:
        Run Date:                          Jun 01 07:02:45
        Passed:                                  1 test(s)
        Skipped:                                 0 test(s)
        Failed:                                  0 test(s)
-----------------------------------------------------------------
Timing Results:
        Test                                 Time (sec)
        -----------------------------------------------
        setup                                      0.00
        085-sudo-cgexec_systemd_scope.py           9.19
        teardown                                   0.00
        -----------------------------------------------
        Total Run Time                             9.19

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