LU-2836 quota: improve test_3 & test_6 of s-q

LU-2836 quota: improve test_3 & test_6 of s-q

When approaching quota limit, client turns to sync write, then the
tests which assume io can be finished quickly could fail, we need
to enlarge the time margin to make sure those tests can pass on slow
system.

Signed-off-by: Niu Yawei <yawei.niu@intel.com>
Change-Id: Ib14a905344eb78cdbd0cd79e3bfd8e50ab21a4d8
Reviewed-on: http://review.whamcloud.com/5539
Tested-by: Hudson
Reviewed-by: Johann Lombardi <johann.lombardi@intel.com>
Tested-by: Maloo <whamcloud.maloo@gmail.com>
Reviewed-by: Fan Yong <fan.yong@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>