selftests/resctrl: Fix resctrl_tests' return code to work with selftest framework

In kselftest framework, if a sub test can not run by some reasons,
the test result should be marked as SKIP rather than FAIL.
Return KSFT_SKIP(4) instead of KSFT_FAIL(1) if resctrl_tests is not run
as root or it is run on a test environment which does not support resctrl.

 - ksft_exit_fail_msg(): returns KSFT_FAIL(1)
 - ksft_exit_skip(): returns KSFT_SKIP(4)

Reviewed-by: Shuah Khan <skhan@linuxfoundation.org>
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>