MDL-74413 phpunit: Move more tests to use correct names and namespaces

Applied the following changes to various testcase classes:

- Namespaced with component[\level2-API]
- Moved to level2-API subdirectory when required.
- Fixed incorrect use statements with leading backslash.
- Remove file phpdoc block
- Remove MOODLE_INTERNAL if not needed.
- Changed code to point to global scope when needed.
- Fix some relative paths and comments here and there.
- All them passing individually.
- Complete runs passing too.

Special mention to:

- Moved to the level2 "privacy" namespace:
  - \mod_assign\privacy\feedback_legacy_polyfill_test
  - \mod_assign\privacy\submission_legacy_polyfill_test

- Moved to the level2 "task" namespace:
  - \core_message\task\migrate_message_data_test
  - \ltiservice_gradebookservices\task\cleanup_test
  - \message_email\task\send_email_test
  - \mod_lti\task\clean_access_tokens_test
  - \mod_workshop\task\cron_task_test

- Moved to the level2 "event" namespace:
  - \core_h5p\event\deleted_test
  - \core_h5p\event\viewed_test

- Renamed to a better name:
  - backup_forum_activity_task_test.php (missing "task")

message/tests/api_test.php

@@ -32,7 +32,7 @@ require_once($CFG->dirroot . '/message/tests/messagelib_test.php');
  * @copyright 2016 Mark Nelson <markn@moodle.com>
  * @license http://www.gnu.org/copyleft/gpl.html GNU GPL v3 or later
  */
-class api_test extends \core_message_messagelib_testcase {
+class api_test extends messagelib_test {
 
     public function test_mark_all_read_for_user_touser() {
         $sender = $this->getDataGenerator()->create_user(array('firstname' => 'Test1', 'lastname' => 'User1'));