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/progress_display_test.php

@@ -28,7 +28,7 @@ class progress_display_test extends \advanced_testcase {
     /**
      * Test basic function of progress_display, updating status and outputting wibbler.
      */
-    public function test_progress_display_update() {
+    public function test_progress_display_update(): void {
         ob_start();
         $progress = new core_mock_progress_display();
         $progress->start_progress('');
@@ -50,7 +50,7 @@ class progress_display_test extends \advanced_testcase {
     /**
      * Test wibbler states. Wibbler should reverse direction at the start and end of its sequence.
      */
-    public function test_progress_display_wibbler() {
+    public function test_progress_display_wibbler(): void {
         ob_start();
         $progress = new core_mock_progress_display();
         $progress->start_progress('');