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

@@ -215,6 +215,7 @@ class phpunit_util extends testing_util {
         get_message_processors(false, true, true);
         filter_manager::reset_caches();
         core_filetypes::reset_caches();
+        core_user::reset_caches();
 
         // Reset static unit test options.
         if (class_exists('\availability_date\condition', false)) {