LU-2826 tests: Re-enable sanity/18

Due to ZFS work in LU-2449, sanity/18 should now work correctly.

Test-Parameters: mdtfilesystemtype=zfs mdsfilesystemtype=zfs  ostfilesystemtype=zfs testlist=sanity
Signed-off-by: Nathaniel Clark <nathaniel.l.clark@intel.com>
Change-Id: I5d9d986b00a36a2b5f30ef9b4ff3d52779070498
Reviewed-on: http://review.whamcloud.com/5720
Tested-by: Hudson
Tested-by: Maloo <whamcloud.maloo@gmail.com>
Reviewed-by: Jian Yu <jian.yu@intel.com>
Reviewed-by: Alex Zhuravlev <alexey.zhuravlev@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>