Replace Codeception\Util\Stub with Codeception\Stub in tests

Because Codeception\Util\Stub will be removed from Codeception 5.0

tests/unit/Codeception/Module/AssertsTest.php

@@ -5,7 +5,7 @@
 use Codeception\Lib\ModuleContainer;
 use Codeception\Module\Asserts;
 use Codeception\PHPUnit\TestCase;
-use Codeception\Util\Stub;
+use Codeception\Stub;
 use Exception;
 use PHPUnit\Framework\AssertionFailedError;
 use RuntimeException;