MDL-73278 phpunit: Rename already namespaced classes to match file name

These are the only cases 100% safe to apply the renaming of the
testcase class names to match the file names.

All other cases are not safe, because they are missing namespace
and may enter into name conflicts. Adding namespaces is not as
simple as imagined because it implies to, also, add a good number
of modifications to core. See the issue for more details.

lib/xapi/tests/external/post_statement_test.php

@@ -48,7 +48,7 @@ require_once($CFG->dirroot . '/webservice/tests/helpers.php');
  * @copyright  2020 Ferran Recio
  * @license    http://www.gnu.org/copyleft/gpl.html GNU GPL v3 or later
  */
-class post_statement_testcase extends externallib_advanced_testcase {
+class post_statement_test extends externallib_advanced_testcase {
 
     /** @var test_helper for generating valid xapi statements. */
     private $testhelper;