[8.x] Adds an ability for the Wormhole class to take us back to the present.

[ambengers]

This PR upgrades the Wormhole to take us back to the present with its API

use Illuminate\Foundation\Testing\Wormhole;
use Illuminate\Support\Carbon;

(new Wormhole(10))->days();

// Currently, to go back to the present we we'll need to reach out for Carbon class like so..
Carbon::setTestNow();

// This PR upgrades the Wormhole to take us back to the present..
Wormhole::back();