LU-11661 test: improve sanityn test_47g

LU-11661 test: improve sanityn test_47g

'stat' may be run before 'mkdir', to avoid this, sync data before
test and wait longer time after 'mkdir' in background.

Test-Parameters: trivial testlist=sanityn,sanityn,sanityn,sanityn mdtfilesystemtype=zfs mdscount=2 mdtcount=4 envdefinitions=ONLY=47g
Signed-off-by: Lai Siyao <lai.siyao@whamcloud.com>
Change-Id: I314bc9d36629a5185efc5ef8281a03337ea77776
Reviewed-on: https://review.whamcloud.com/33647
Tested-by: Jenkins
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Patrick Farrell <paf@cray.com>
Reviewed-by: Wei Liu <sarah@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>