LU-2189 tests: ensure sync stability before sanityn/36

Encourage data to be fully sync'd before getting new measurements.

Test-Parameters: testlist=sanityn  mdtfilesystemtype=zfs mdsfilesystemtype=zfs ostfilesystemtype=zfs
Signed-off-by: Nathaniel Clark <nathaniel.l.clark@intel.com>
Change-Id: I42be5714b59cd4c22c65d7524f2b71ec0a07dfa4
Reviewed-on: http://review.whamcloud.com/5722
Tested-by: Hudson
Tested-by: Maloo <whamcloud.maloo@gmail.com>
Reviewed-by: Jian Yu <jian.yu@intel.com>
Reviewed-by: Li Wei <wei.g.li@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>