selftests/resctrl: Make resctrl_tests run using kselftest framework

In kselftest framework, all tests can be build/run at a time,
and a sub test also can be build/run individually. As follows:
$ make kselftest-all TARGETS=resctrl
$ make -C tools/testing/selftests run_tests
$ make -C tools/testing/selftests TARGETS=resctrl run_tests

However, resctrl_tests cannot be run using kselftest framework,
users have to change directory to tools/testing/selftests/resctrl/,
run "make" to build executable file "resctrl_tests",
and run "sudo ./resctrl_tests" to execute the test.

To build/run resctrl_tests using kselftest framework.
Modify tools/testing/selftests/Makefile
and tools/testing/selftests/resctrl/Makefile.

Even after this change, users can still build/run resctrl_tests
without using framework as before.

Reviewed-by: Reinette Chatre <reinette.chatre@intel.com> # resctrl changes
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>