Add missing use statement

Author: jmikola

tests/Ticket/GH2783Test.php

@@ -6,6 +6,7 @@
 
 use MongoDB\Laravel\Eloquent\Model;
 use MongoDB\Laravel\Relations\MorphTo;
+use MongoDB\Laravel\Tests\TestCase;
 use Illuminate\Database\Eloquent\Relations\MorphOne;
 
 /**