LU-4176 tests: re-enable sanity-hsm/test_31a

LU-4176 tests: re-enable sanity-hsm/test_31a

Due to frequent failures and despite first patch/fix for LU-4176 has
landed, sanity-hsm/test_31a has been disabled like other sub-tests
in a patch for LU-4178. Since both test_31[b,c], which also triggered
the same issue, are running fine now and with original fix for
LU-4176, it is time to re-enable it.

Signed-off-by: Bruno Faccini <bruno.faccini@intel.com>
Change-Id: I5cc6c23b7cb48e190438eb3b84fa55ffdb198739
Reviewed-on: http://review.whamcloud.com/9577
Tested-by: Jenkins
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: James Simmons <uja.ornl@gmail.com>