Added test stubs

tests/Stubs/UserEntity.php

@@ -0,0 +1,13 @@
+<?php
+
+namespace LeagueTests\Stubs;
+
+use League\OAuth2\Server\Entities\Interfaces\UserEntityInterface;
+
+class UserEntity implements UserEntityInterface
+{
+    public function getIdentifier()
+    {
+        return 123;
+    }
+}