LU-8565 test: change sanity 255a to not fail for performance when running in VM

LU-8565 test: change sanity 255a to not fail for performance when running in VM

Considering we may run our testing in VMs with other parallel workloads,
and also out VMs are short on memory. Therefore the complete time of I/O
task is unreliable and depends on the workload on the host machine when
the task is running.
So as Andreas suggested, here we change sanity 255a to not fail even if
the performance isn't as expected when running in a VM, like we did to
sanity 248.

Test-Parameters: trivial

Change-Id: If2a76c64f053dc6c7dc8acf4afd5a68ea3a757b6
Signed-off-by: Gu Zheng <gzheng@ddn.com>
Reviewed-on: http://review.whamcloud.com/22375
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>