MDL-77837 phpunit: Ensure that the cron user setter is used

When running an adhoc task in a unit test we should use the cron variant
of the set user method to mimic the behaviour of a real cron run.

lib/phpunit/tests/advanced_test.php

@@ -23,8 +23,13 @@ namespace core;
  * @category   test
  * @copyright  2012 Petr Skoda {@link http://skodak.org}
  * @license    http://www.gnu.org/copyleft/gpl.html GNU GPL v3 or later
+ * @coversDefaultClass \advanced_testcase
  */
 class advanced_test extends \advanced_testcase {
+    public static function setUpBeforeClass(): void {
+        global $CFG;
+        require_once(__DIR__ . '/fixtures/adhoc_test_task.php');
+    }
 
     public function test_debugging() {
         global $CFG;
@@ -697,4 +702,53 @@ class advanced_test extends \advanced_testcase {
         self::resetAllData(false);
         self::assertFalse(\core_useragent::get_user_agent_string(), 'It should not be set again, data was reset.');
     }
+
+    /**
+     * @covers ::runAdhocTasks
+     */
+    public function test_runadhoctasks_no_tasks_queued(): void {
+        $this->runAdhocTasks();
+        $this->expectOutputRegex('/^$/');
+    }
+
+    /**
+     * @covers ::runAdhocTasks
+     */
+    public function test_runadhoctasks_tasks_queued(): void {
+        $this->resetAfterTest(true);
+        $admin = get_admin();
+        \core\task\manager::queue_adhoc_task(new \core_phpunit\adhoc_test_task());
+        $this->runAdhocTasks();
+        $this->expectOutputRegex("/Task was run as {$admin->id}/");
+    }
+
+    /**
+     * @covers ::runAdhocTasks
+     */
+    public function test_runadhoctasks_with_existing_user_change(): void {
+        $this->resetAfterTest(true);
+        $admin = get_admin();
+
+        $this->setGuestUser();
+        \core\task\manager::queue_adhoc_task(new \core_phpunit\adhoc_test_task());
+        $this->runAdhocTasks();
+        $this->expectOutputRegex("/Task was run as {$admin->id}/");
+    }
+
+    /**
+     * @covers ::runAdhocTasks
+     */
+    public function test_runadhoctasks_with_existing_user_change_and_specified(): void {
+        global $USER;
+
+        $this->resetAfterTest(true);
+        $user = $this->getDataGenerator()->create_user();
+
+        $this->setGuestUser();
+        $task = new \core_phpunit\adhoc_test_task();
+        $task->set_userid($user->id);
+        \core\task\manager::queue_adhoc_task($task);
+        $this->runAdhocTasks();
+        $this->expectOutputRegex("/Task was run as {$user->id}/");
+    }
 }