[8.x] Add flush handler to output buffer for streamed test response (bugfix) (#42481)

* Add handler to catch ob_flush

* Make formatter happy.

Author: nagmat84

src/Illuminate/Testing/TestResponse.php

@@ -1491,11 +1491,17 @@ public function streamedContent()
             PHPUnit::fail('The response is not a streamed response.');
         }
 
-        ob_start();
+        ob_start(function (string $buffer): string {
+            $this->streamedContent .= $buffer;
+
+            return '';
+        });
 
         $this->sendContent();
 
-        return $this->streamedContent = ob_get_clean();
+        ob_end_clean();
+
+        return $this->streamedContent;
     }
 
     /**