LU-14279 test: fix block soft testing failure

LU-14279 test: fix block soft testing failure

Soft least qunit was introduced to avoid performance
drop when users have reached soft limit, but timer has
not reached, it tried to acquire more space(not more than
least qunit) to get reasonable performance.

Test cases need be aware of this, which means slave might
exceed quota limit a bit(but should not more than least qunit
eg 4M).

Test-Parameters: trivial testlist=sanity-quota env=ONLY="3a 3b 3c"
Signed-off-by: Wang Shilong <wshilong@ddn.com>
Change-Id: Ia221d97d158a8da4dc1fe1611aebac2f5086440e
Reviewed-on: https://review.whamcloud.com/41094
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Hongchao Zhang <hongchao@whamcloud.com>
Reviewed-by: Sergey Cheremencev <sergey.cheremencev@hpe.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>