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/tests/encryption_test.php

@@ -31,7 +31,7 @@ namespace core;
  * @copyright 2020 The Open University
  * @license http://www.gnu.org/copyleft/gpl.html GNU GPL v3 or later
  */
-class encryption_testcase extends \basic_testcase {
+class encryption_test extends \basic_testcase {
 
     /**
      * Clear junk created by tests.