[5.5] flush listeners of custom eloquent events

[themsaid]

Having this in your model:

class User extends Model
{
    protected $dispatchesEvents = ['created' => UserCreated::class];
}

Currently if you do this in your tests:

public function testFlushEventListeners()
    {
        User::flushEventListeners();
        
        $user = factory(User::class)->create();
    }

The listeners of the UserCreated are still going to be fired, this PR fixes that.