MDL-81698 phpunit: Apply various fixes towards 1by1 execution

The changes here are heterogeneous:
- Include stuff that is not available (other test has included it).
  Sometimes local to a unit test, others in setupBeforeClass() or
  globally, ... depends on every case.
- Rename some tests (namespaces, test name, ...) towards getting it
  running.
- Amend small bits here and there.

Important note: I've left any "cosmetic" warning out from the
changes, only a few errors (like long array syntax) have been fixed.

lib/tests/task/manager_test.php

@@ -16,6 +16,14 @@
 
 namespace core\task;
 
+defined('MOODLE_INTERNAL') || die();
+
+// We need to keep this here because there is a provider
+// needing \core\task\adhoc_test_task and cannot move it
+// to setUpBeforeClass() or similar. Whenever we allow to
+// autoload fixtures, this can be removed.
+require_once(__DIR__ . '/../fixtures/task_fixtures.php');
+
 /**
  * This file contains the unit tests for the task manager.
  *
@@ -26,10 +34,6 @@ namespace core\task;
  * @covers \core\task\manager
  */
 final class manager_test extends \advanced_testcase {
-    public static function setUpBeforeClass(): void {
-        require_once(__DIR__ . '/../fixtures/task_fixtures.php');
-    }
-
     /**
      * Data provider for test_get_candidate_adhoc_tasks.
      *