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/tests/task/adhoc_task_test.php

@@ -921,8 +921,7 @@ final class adhoc_task_test extends \advanced_testcase {
     /**
      * Test send messages when adhoc task reaches the max fail delay time.
      *
-     * @covers ::adhoc_task_failed
-     * @covers ::send_failed_task_max_delay_message
+     * @covers \core\task\failed_task_callbacks::send_failed_task_max_delay_message
      */
     public function test_adhoc_message_max_fail_delay(): void {
         $this->resetAfterTest();