selftests/resctrl: Use correct exit code when tests fail

Use ksft_finished() after running tests so that resctrl_tests doesn't
return exit code 0 when tests fail.

Consequently, report the MBA and MBM tests as skipped when running on
non-Intel hardware, otherwise resctrl_tests will exit with a failure
code.

Signed-off-by: Peter Newman <peternewman@google.com>
Reviewed-by: Shaopeng Tan <tan.shaopeng@fujitsu.com>
Tested-by: Shaopeng Tan <tan.shaopeng@fujitsu.com>
Reviewed-by: Reinette Chatre <reinette.chatre@intel.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>