selftests/resctrl: Reduce failures due to outliers in MBA/MBM tests

The initial value of 5% chosen for the maximum allowed percentage
difference between resctrl mbm value and IMC mbm value in

commit 06bd03a57f8c ("selftests/resctrl: Fix MBA/MBM results reporting
       format") was "randomly chosen value" (as admitted by the changelog).

When running tests in our lab across a large number platforms, 5%
difference upper bound for success seems a bit on the low side for the
MBA and MBM tests. Some platforms produce outliers that are slightly
above that, typically 6-7%, which leads MBA/MBM test frequently
failing.

Replace the "randomly chosen value" with a success bound that is based
on those measurements across large number of platforms by relaxing the
MBA/MBM success bound to 8%. The relaxed bound removes the failures due
the frequent outliers.

Fixed commit description style error during merge:
Shuah Khan <[email protected]>

Fixes: 06bd03a57f8c ("selftests/resctrl: Fix MBA/MBM results reporting format")
Signed-off-by: Ilpo Järvinen <[email protected]>
Tested-by: Shaopeng Tan <[email protected]>
Reviewed-by: Reinette Chatre <[email protected]>
Reviewed-by: Shaopeng Tan <[email protected]>
Cc: <[email protected]>
Signed-off-by: Shuah Khan <[email protected]>

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