Update NotificationFake with notifications getter (laravel#43558)

When testing notifications this is super handy for testing whether the correct notifications
have been sent. Similar method is available in the QueueFake with `Queue::getJobs()`

src/Illuminate/Support/Testing/Fakes/NotificationFake.php

@@ -348,4 +348,14 @@ public function locale($locale)
 
         return $this;
     }
+
+    /**
+     * Get the notifications that have been sent.
+     *
+     * @return array
+     */
+    public function sentNotifications()
+    {
+        return $this->notifications;
+    }
 }