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/gdlib_test.php

@@ -32,7 +32,7 @@ class gdlib_test extends \basic_testcase {
         $this->fixturepath = __DIR__ . DIRECTORY_SEPARATOR . 'fixtures' . DIRECTORY_SEPARATOR;
     }
 
-    public function test_generate_image_thumbnail() {
+    public function test_generate_image_thumbnail(): void {
         global $CFG;
         require_once($CFG->libdir . '/gdlib.php');
 
@@ -50,7 +50,7 @@ class gdlib_test extends \basic_testcase {
         $this->assertEquals('image/png', $imageinfo['mime']);
     }
 
-    public function test_generate_image_thumbnail_from_string() {
+    public function test_generate_image_thumbnail_from_string(): void {
         global $CFG;
         require_once($CFG->libdir . '/gdlib.php');
 
@@ -77,7 +77,7 @@ class gdlib_test extends \basic_testcase {
         $this->assertEquals('image/png', $imageinfo['mime']);
     }
 
-    public function test_resize_image() {
+    public function test_resize_image(): void {
         global $CFG;
         require_once($CFG->libdir . '/gdlib.php');
 
@@ -108,7 +108,7 @@ class gdlib_test extends \basic_testcase {
         $this->assertEquals('image/png', $imageinfo['mime']);
     }
 
-    public function test_resize_image_from_image() {
+    public function test_resize_image_from_image(): void {
         global $CFG;
         require_once($CFG->libdir . '/gdlib.php');