MDL-81456 core: Use DI for all hook access

Using DI for all hook access means that it becomes significantly easier
to mock hooks and callbacks for unit testing without fundamentally
altering the structure of the code purely for the purposes of unit
testing.

lib/phpunit/classes/util.php

@@ -23,6 +23,11 @@
  * @license    http://www.gnu.org/copyleft/gpl.html GNU GPL v3 or later
  */
 
+use core\{
+    di,
+    hook,
+};
+
 require_once(__DIR__.'/../../testing/classes/util.php');
 require_once(__DIR__ . "/coverage_info.php");
 
@@ -105,7 +110,7 @@ class phpunit_util extends testing_util {
         global $DB, $CFG, $USER, $SITE, $COURSE, $PAGE, $OUTPUT, $SESSION, $FULLME, $FILTERLIB_PRIVATE;
 
         // Stop all hook redirections.
-        \core\hook\manager::get_instance()->phpunit_stop_redirections();
+        di::get(hook\manager::class)->phpunit_stop_redirections();
 
         // Reset the hook manager instance.
         \core\hook\manager::phpunit_reset_instance();