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 06bd03a57f ("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 <skhan@linuxfoundation.org>

Fixes: 06bd03a57f ("selftests/resctrl: Fix MBA/MBM results reporting format")
Signed-off-by: Ilpo Järvinen <ilpo.jarvinen@linux.intel.com>
Tested-by: Shaopeng Tan <tan.shaopeng@jp.fujitsu.com>
Reviewed-by: Reinette Chatre <reinette.chatre@intel.com>
Reviewed-by: Shaopeng Tan <tan.shaopeng@jp.fujitsu.com>
Cc: <stable@vger.kernel.org>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>

tools/testing/selftests/resctrl/mba_test.c

@@ -12,7 +12,7 @@
 
 #define RESULT_FILE_NAME	"result_mba"
 #define NUM_OF_RUNS		5
-#define MAX_DIFF_PERCENT	5
+#define MAX_DIFF_PERCENT	8
 #define ALLOCATION_MAX		100
 #define ALLOCATION_MIN		10
 #define ALLOCATION_STEP		10

tools/testing/selftests/resctrl/mbm_test.c

@@ -11,7 +11,7 @@
 #include "resctrl.h"
 
 #define RESULT_FILE_NAME	"result_mbm"
-#define MAX_DIFF_PERCENT	5
+#define MAX_DIFF_PERCENT	8
 #define NUM_OF_RUNS		5
 
 static int