LU-6313 tests: more robust for scrub test_11

LU-6313 tests: more robust for scrub test_11

For the sanity-scrub test_11, except for the known created by the
test scripts, there may be other objects (such as for llog) have
been created before the first OI scrub scaning. So it is not easy
to estimate how many objects should be skipped during the first
OI scrub scanning. So we only check that the number of skipped
files is more than the number or known created.

Signed-off-by: Fan Yong <fan.yong@intel.com>
Change-Id: Iced9fb255559394117880514c5e716d05a81a177
Reviewed-on: http://review.whamcloud.com/13957
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Jian Yu <jian.yu@intel.com>
Reviewed-by: Lai Siyao <lai.siyao@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>