docs: add namespace to sample code in testing

Author: kenjis

user_guide_src/source/testing/overview/017.php

@@ -1,7 +1,10 @@
 <?php
 
+namespace Tests;
+
 use CodeIgniter\Config\Factories;
 use CodeIgniter\Test\CIUnitTestCase;
+use Tests\Support\Mock\MockUserModel;
 
 final class SomeTest extends CIUnitTestCase
 {

user_guide_src/source/testing/overview/021.php

@@ -1,5 +1,7 @@
 <?php
 
+namespace Tests;
+
 use CodeIgniter\I18n\Time;
 use CodeIgniter\Test\CIUnitTestCase;