LU-8973 tests: in sanity 103a use $tdir instead of $MOUNT

LU-8973 tests: in sanity 103a use $tdir instead of $MOUNT

In sanity.sh test_103a we should use $tdir instead of $MOUNT
in case of any unclean testing directory.

Test-Parameters: trivial
Signed-off-by: Emoly Liu <emoly.liu@intel.com>
Change-Id: I891bbc0aab28bb58589b66405e0ad5c40fdad25f
Reviewed-on: https://review.whamcloud.com/24518
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Yang Sheng <yang.sheng@intel.com>
Reviewed-by: Bob Glossman <bob.glossman@intel.com>
Reviewed-by: Jian Yu <jian.yu@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>