test: fix Creation of dynamic property error

ErrorException: Creation of dynamic property class@anonymous::$id is deprecated

Author: kenjis

tests/system/Database/Live/GetTest.php

@@ -229,7 +229,15 @@ public function testGetRowWithReturnType()
 
     public function testGetRowWithCustomReturnType()
     {
-        $testClass = new class () {};
+        $testClass = new class () {
+            public $id;
+            public $name;
+            public $email;
+            public $country;
+            public $created_at;
+            public $updated_at;
+            public $deleted_at;
+        };
 
         $user = $this->db->table('user')->get()->getRow(0, get_class($testClass));