LU-8427 build: clear file lustre-tests.files

LU-8427 build: clear file lustre-tests.files

Clear lustre-tests.files before append to it

Test-Parameters: trivial

Change-Id: I1b916b0b68b9bb57c543daf71892947ee896c8da
Signed-off-by: Dmitry Eremin <dmitry.eremin@intel.com>
Reviewed-on: http://review.whamcloud.com/21229
Tested-by: Jenkins
Reviewed-by: Bob Glossman <bob.glossman@intel.com>
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Christopher J. Morrone <morrone2@llnl.gov>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>