LU-4613 tests: purge older request result in test_12o

LU-4613 tests: purge older request result in test_12o

sanity-hsm/test_12o sub-test, which has been introduced as part
of LU-3834, submits 2 RESTORE requests for the same FID and thus
needs to purge 1st result from log before to check 2nd.

Signed-off-by: Bruno Faccini <bruno.faccini@intel.com>
Change-Id: Ia2a0ead487b29a68c8a920bae2aa1d654eac4051
Reviewed-on: http://review.whamcloud.com/9295
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Aurelien Degremont <aurelien.degremont@cea.fr>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>