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/grades_external_test.php

@@ -134,7 +134,7 @@ class grades_external_test extends \externallib_advanced_testcase {
     /**
      * Test update_grades()
      */
-    public function test_update_grades() {
+    public function test_update_grades(): void {
         global $DB;
 
         $this->resetAfterTest(true);