Document that somewhere along the line laravel started respecting APP_URL or app.url in tests.

| Q | A
| ----------------- | ---
| Bug? | no
| New Feature? | no
| Framework | Lumen
| Framework version | 5.5
| Package version | 1.x.y
| PHP version | 7.0

#### Actual Behaviour
#### Expected Behaviour

See somewhere in the documentation that APP_URL is now respected when using TestCase::post etc.
I upgraded from 5.3 and all my tests showed 404.
I assume this is related to a lot of the 404 issues within phpunit.

#### Steps to Reproduce

I you set `config/app.php`s `url` key, it'll be used if you do the following in a `TestCase`:
```php
$this->post('/some/path', ['test' => 'toast']);
```
Resulting in a request /some/path.