LU-6385 tests: sync I/O in obdfilter-survey

LU-6385 tests: sync I/O in obdfilter-survey

This ensures I/O is synced before each test.  Previous patch
http://review.whamcloud.com/14143 works only with the Lustre test
framework.  This will ensure I/O is always sysnced.

Signed-off-by: Nathaniel Clark <nathaniel.l.clark@intel.com>
Change-Id: Ib19a9e4afd8ca83deceb78fe8fdb4d231da0bc40
Reviewed-on: http://review.whamcloud.com/16942
Tested-by: Jenkins
Reviewed-by: Alex Zhuravlev <alexey.zhuravlev@intel.com>
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Bob Glossman <bob.glossman@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>