ftests-999: fix random dir name generation logic

ftests-999: fix random dir name generation logic

The current logic of creating a random directory under the /tmp/
the directory is incorrect because the random directory is generated
under '/tmp/999stress', making it a common parent directory and
attempting to create/delete it will fail the test instance, that
overlaps. Fix this by generating '/tmp/<random-999stress>' making
unique per run.

-----------------------------------------------------------------
Test Results:
        Run Date:                          Jun 12 11:39:41
        Passed:                                  1 test(s)
        Skipped:                                 0 test(s)
        Failed:                                  0 test(s)
-----------------------------------------------------------------
Timing Results:
        Test                          Time (sec)
        ----------------------------------------
        setup                               0.00
        999-stress-cgroup_init.py           1.55
        teardown                            0.00
        ----------------------------------------
        Total Run Time                      1.55

Fixes: b81915a1fbe7 ("tests-999: add a random directory to mount path")
Signed-off-by: Kamalesh Babulal <kamalesh.babulal@oracle.com>
Signed-off-by: Tom Hromatka <tom.hromatka@oracle.com>