MDL-50705 core_user: introduce new fill_properties_cache()

Also the get_property_definition() was created to get the property without retrieve the whole definition cache and
    unit tests were created to tests those new methods.

lib/phpunit/classes/util.php

@@ -204,6 +204,7 @@ class phpunit_util extends testing_util {
         filter_manager::reset_caches();
         // Reset internal users.
         core_user::reset_internal_users();
+        core_user::reset_caches();
 
         //TODO MDL-25290: add more resets here and probably refactor them to new core function