[11.x] Http Client: fake connection exception

[gdebrauwer]

Currently it is possible to fake every possible response when faking the http client in a test, but it is currently not possible to fake a connection exception. This PR makes that possible.

try {
    Http::get('https://laravel.com')
} catch (ConnectionException) {
    // Some special handling that you want to test.
}

I made this possible by providing an easy way to create failed connection similar to creating a fake response:

Http::fake(['https://laravel.com' => Http::failedConnection()]);

// Similar syntax to:
Http::fake(['https://laravel.com' => Http::response()]);

I ensured faking a connection exception works in all different ways you can currently create fake responses (e.g. closure, response sequence) and I added a test for each one.