selftests/resctrl: Move cat_val() to cat_test.c and rename to cat_test()

The main CAT test function is called cat_val() and resides in cache.c
which is illogical.

Rename the function to cat_test() and move it into cat_test.c.

Signed-off-by: Ilpo Järvinen <ilpo.jarvinen@linux.intel.com>
Reviewed-by: Reinette Chatre <reinette.chatre@intel.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>