MDL-81522 phpunit: Add missing void return type to all tests

While this change is not 100% required now, it's good habit
and we are checking for it since Moodle 4.4.

All the changes in this commit have been applied automatically
using the moodle.PHPUnit.TestReturnType sniff and are, exclusively
adding the ": void" return types when missing.

lib/tests/authlib_test.php

@@ -26,7 +26,7 @@ namespace core;
  * @license    http://www.gnu.org/copyleft/gpl.html GNU GPL v3 or later
  */
 class authlib_test extends \advanced_testcase {
-    public function test_lockout() {
+    public function test_lockout(): void {
         global $CFG;
         require_once("$CFG->libdir/authlib.php");
 
@@ -110,7 +110,7 @@ class authlib_test extends \advanced_testcase {
         ini_set('error_log', $oldlog);
     }
 
-    public function test_authenticate_user_login() {
+    public function test_authenticate_user_login(): void {
         global $CFG;
 
         $this->resetAfterTest();
@@ -436,7 +436,7 @@ class authlib_test extends \advanced_testcase {
         unset($CFG->enableloginrecaptcha);
     }
 
-    public function test_user_loggedin_event_exceptions() {
+    public function test_user_loggedin_event_exceptions(): void {
         try {
             $event = \core\event\user_loggedin::create(array('objectid' => 1));
             $this->fail('\core\event\user_loggedin requires other[\'username\']');