Fix integration tests compatibility

[MAILPOET-4001]

tests/integration/Config/InstallerTest.php

@@ -9,7 +9,9 @@ use MailPoet\Config\Installer;
 use MailPoet\WP\Functions as WPFunctions;
 
 class InstallerTest extends \MailPoetTest {
+  /** @var Installer */
   public $installer;
+  /** @var string */
   public $slug;
 
   public function _before() {

tests/integration/Config/RendererTest.php

@@ -9,6 +9,7 @@ use MailPoet\Config\Renderer;
 use MailPoetVendor\Twig\Environment as TwigEnvironment;
 
 class RendererTest extends \MailPoetTest {
+  /** @var Renderer */
   public $renderer;
 
   public function _before() {

tests/integration/Config/UpdaterTest.php

@@ -7,9 +7,13 @@ use Codeception\Stub\Expected;
 use MailPoet\Config\Updater;
 
 class UpdaterTest extends \MailPoetTest {
+  /** @var Updater */
   public $updater;
+  /** @var string */
   public $version;
+  /** @var string */
   public $slug;
+  /** @var string */
   public $pluginName;
 
   public function _before() {