[HttpFoundation] Fix deprecation in tests on PHP 8.5

Author: nicolas-grekas

src/Symfony/Component/HttpFoundation/Tests/StreamedResponseTest.php

@@ -32,7 +32,7 @@ public function testConstructorWithChunks()
 
         $buffer = '';
         ob_start(function (string $chunk) use (&$buffer) {
-            $buffer .= $chunk;
+            return $buffer .= $chunk;
         });
         $callback();