LU-13297 tests: parallel-scale enhancement

Patch changes parallel-scale tests to use t-f test_mkdir()
instead of mkdir to have the possibility to run these tests on
striped directories.

Test-Parameters: trivial testlist=parallel-scale,parallel-scale-nfsv3
Cray-bug-id: LUS-8291
Signed-off-by: Elena Gryaznova <elena.gryaznova@hpe.com>
Reviewed-by: Alexander Boyko <alexander.boyko@hpe.com>
Reviewed-by: Vladimir Saveliev <c17830@cray.com>
Change-Id: I6a0d52d7115668ef2bc7397a9a1012dbcb9e0526
Reviewed-on: https://review.whamcloud.com/37732
Reviewed-by: Alexey Lyashkov <alexey.lyashkov@hpe.com>
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>