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.

question/engine/tests/questionusagebyactivity_data_test.php

@@ -35,7 +35,7 @@ require_once(__DIR__ . '/helpers.php');
  * @license   http://www.gnu.org/copyleft/gpl.html GNU GPL v3 or later
  */
 class questionusagebyactivity_data_test extends \data_loading_method_test_base {
-    public function test_load() {
+    public function test_load(): void {
         $scid = \context_system::instance()->id;
         $records = new question_test_recordset(array(
         array('qubaid', 'contextid', 'component', 'preferredbehaviour',
@@ -90,7 +90,7 @@ class questionusagebyactivity_data_test extends \data_loading_method_test_base {
         $this->assertEquals(array('-finish' => '1'), $step->get_all_data());
     }
 
-    public function test_load_data_no_steps() {
+    public function test_load_data_no_steps(): void {
         // The code had a bug where if one question_attempt had no steps,
         // load_from_records got stuck in an infinite loop. This test is to
         // verify that no longer happens.
@@ -122,7 +122,7 @@ class questionusagebyactivity_data_test extends \data_loading_method_test_base {
         $this->assertEquals(0, $qa->get_num_steps());
     }
 
-    public function test_load_data_no_qas() {
+    public function test_load_data_no_qas(): void {
         // The code had a bug where if a question_usage had no question_attempts,
         // load_from_records got stuck in an infinite loop. This test is to
         // verify that no longer happens.