LU-3350 tests: allow test-error for lfsck/scrub speed

LU-3350 tests: allow test-error for lfsck/scrub speed

Affected by the test environment and CPU schedule randomness,
especailly test in virtal machine, it is not easy to control
the lfsck/scrub speed accurtely. So we have to consider more
test-error.

Test-Parameters: testlist=sanity-scrub,sanity-lfsck

Signed-off-by: Fan Yong <fan.yong@intel.com>
Change-Id: I794d26d2bfa73d7000e77790cc7a2c8203f7db06
Reviewed-on: http://review.whamcloud.com/6374
Tested-by: Hudson
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Tested-by: Maloo <whamcloud.maloo@gmail.com>
Reviewed-by: Jian Yu <jian.yu@intel.com>