Add the return type in the TestClient in the testsuite

This avoids a deprecation warning (and makes that class ready for
Symfony 6)

Author: stof

tests/Custom/ErrorHandlingTest.php

@@ -175,7 +175,7 @@ public function setNextScript($script)
         $this->nextScript = $script;
     }
 
-    protected function doRequest($request)
+    protected function doRequest($request): object
     {
         if (null === $this->nextResponse) {
             return new Response();