MDL-82207 phpunit: fix various @covers annotations (take#1)

This fixes various (not all) wrong @covers annotations that
are reported as warnings by PHPUnit when it's run with
code-coverage enabled.

When possible, the preferred solution is to change to
@covers at class level, that is the recommended way.

If multiple classes are involved, then a mix of @coversDefaultClass
and @covers at method level are used (always trying to use the
minimum needed patch).

This is the first of a series of issues created as sub-tasks
of MDL-82142.

lib/testing/tests/testing_generator_test.php

@@ -23,7 +23,7 @@ namespace core;
  * @category   phpunit
  * @copyright  2012 Petr Skoda {@link http://skodak.org}
  * @license    http://www.gnu.org/copyleft/gpl.html GNU GPL v3 or later
- * @covers \core\testing_data_generator
+ * @covers \testing_data_generator
  */
 class testing_generator_test extends \advanced_testcase {
     public function test_get_plugin_generator_good_case(): void {