LU-2562 test: cleanup previous test result

LU-2562 test: cleanup previous test result

in sanity.sh, test_51ba only depends on test_51b, but test_51a
also create files under $DIR/$tdir, which causes test_51ba fail
for it also takes those files into account.

Change-Id: I42f491f64c8d2d65bee656c167c9523274cd7200
Signed-off-by: Hongchao Zhang <hongchao.zhang@whamcloud.com>
Reviewed-on: http://review.whamcloud.com/5183
Tested-by: Hudson
Tested-by: Maloo <whamcloud.maloo@gmail.com>
Reviewed-by: Niu Yawei <yawei.niu@intel.com>
Reviewed-by: Jian Yu <jian.yu@intel.com>
Reviewed-by: Nathaniel Clark <nathaniel.l.clark@intel.com>
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>