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/componentlib_test.php

@@ -44,7 +44,7 @@ require_once($CFG->libdir.'/componentlib.class.php');
  */
 class componentlib_test extends \advanced_testcase {
 
-    public function test_component_installer() {
+    public function test_component_installer(): void {
         global $CFG;
 
         $url = $this->getExternalTestFileUrl('');
@@ -78,7 +78,7 @@ class componentlib_test extends \advanced_testcase {
     /**
      * Test the public API of the {@link lang_installer} class.
      */
-    public function test_lang_installer() {
+    public function test_lang_installer(): void {
 
         // Test the manipulation with the download queue.
         $installer = new testable_lang_installer();