selftests/resctrl: Simplify perf usage in CAT test

selftests/resctrl: Simplify perf usage in CAT test

The CAT test relies on the PERF_COUNT_HW_CACHE_MISSES event to determine if
modifying a cache portion size is successful. This event is configured to
report the data as part of an event group, but no other events are added to
the group.

Remove the unnecessary PERF_FORMAT_GROUP format setting. This eliminates
the need for struct perf_event_read and results in read() of the associated
file descriptor to return just one value associated with the
PERF_COUNT_HW_CACHE_MISSES event of interest.

Link: https://lore.kernel.org/r/fb69325eba5031b735fa79effaaacd797c9c6040.1775266384.git.reinette.chatre@intel.com
Signed-off-by: Reinette Chatre <reinette.chatre@intel.com>
Tested-by: Chen Yu <yu.c.chen@intel.com>
Reviewed-by: Ilpo Järvinen <ilpo.jarvinen@linux.intel.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>