[FrameworkBundle] made so IDEs will catch up right Response class

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | no
| BC breaks?    |  no
| Deprecations? |  no
| Tests pass?   |  yes
| Fixed tickets | #6903 
| License       | MIT
| Doc PR        |  -

When using `FrameworkBundle/Test/WebTestCase` and trying to get a `getResponse()` IDEs catching up `BrowserKit\Response` instead of `HttpFoundation\Response`. This causes false-errors and wrong code completion when accessing
```php
$client = static::createClient();
// ...
$headers = $client->getResponse()->headers
```


- [ ] gather feedback my changes
- [ ] probably there are some annotations magic that can override return class of a method, so nothing will be overloaded