selftests/resctrl: Update README about using kselftest framework to build/run resctrl_tests
resctrl_tests can be built or run using kselftests framework.
Add description on how to do so in README.
Reviewed-by: Reinette Chatre <reinette.chatre@intel.com>
Reviewed-by: Fenghua Yu <fenghua.yu@intel.com>
Signed-off-by: Shaopeng Tan <tan.shaopeng@jp.fujitsu.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>
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