selftests/resctrl: Call kselftest APIs to log test results

Call kselftest APIs instead of using printf() to log test results
for cleaner code and better future extension.

Suggested-by: Shuah Khan <skhan@linuxfoundation.org>
Tested-by: Babu Moger <babu.moger@amd.com>
Signed-off-by: Fenghua Yu <fenghua.yu@intel.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>