ChannelManager doesn't implement any common contract with NotificationFake

[oleghalin]

Currently ChannelManager implements DispatcherContract and FactoryContract

use Illuminate\Contracts\Notifications\Dispatcher as DispatcherContract;
use Illuminate\Contracts\Notifications\Factory as FactoryContract;

If inject it in __construct(ChannelManager $manager) and cover implementation with test which mocks Notification implementation using Notification::fake() PHP will throw exception that NotificationFake is not compatible with requested constructor.

We can inject DispatcherContract but at the same time DispatcherContract doesn't describe implemented locale() method on ChannelManager.php.

It would be very useful to introduce Contract which will implement both DispatcherContract and FactoryContract so we will be able to implement it on mocked and basic ChannelManager.

Let me know if it makes sense, if so i will be happy to make PR for this.