This PR adds an actingAsGuest method to be a complement to the actingAs method in tests. When called, it makes the default or specified guard forget the user that was set to it.

My particular use case was in a test that simulated a webhook call coming in after a user request. While the initial user request would be authenticated, I wanted to be sure the webhook request did not have any user associated with it, as it would not in reality.

// Simulate initial user request
$this->actingAs($myUser)->post(route('buy-thing'));

// Simulate webhook
$this->actingAsGuest()->post(route('handle-webhook'), ['payload' => 'foo']);

// Simulating another step from the user's perspective
$this->actingAs($myUser)->get(route('after-purchase-redirect'));