LU-2874 tests: mark slow sync zfs tests as EXCEPT_SLOW

For tests that take a very long time on ZFS but not on ldiskfs, place
them in EXCEPT_SLOW when running on zfs.

Signed-off-by: Nathaniel Clark <nathaniel.l.clark@intel.com>
Change-Id: I52193d9a1c51e1276a5ac86c46f32e5ba5dd6299
Reviewed-on: http://review.whamcloud.com/5553
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Tested-by: Hudson
Tested-by: Maloo <whamcloud.maloo@gmail.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>