[11.x] Fix assertContent on laravel test that respond with Symfony Response Object

[tben]

This pull request fixes an issue when a controller is returning a Symfony\Component\HttpFoundation\Response object instead of the usual Laravel Response object. This is down to Symfony response object not having content function defined.

I noticed that the rest of the TestResponse class uses ->getContent(), which works with the Symfony object. So I've applied that to this function as well.

Test:

$test = \Illuminate\Testing\TestResponse::fromBaseResponse(
    new \Symfony\Component\HttpFoundation\Response('testing', 200)
 );

$test->assertContent('testing');