Add missing declaration of the client static property for Symfony < 4.3.

Author: gnutix

src/Test/WebTestCase.php

@@ -40,6 +40,9 @@
  */
 abstract class WebTestCase extends BaseWebTestCase
 {
+    /** @var Client|null */
+    protected static $client;
+
     protected $environment = 'test';
 
     protected $containers;