docs: add namespace Tests; to sample code

Author: kenjis

user_guide_src/source/testing/overview/018.php

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

user_guide_src/source/testing/overview/019.php

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

user_guide_src/source/testing/overview/020.php

@@ -1,5 +1,7 @@
 <?php
 
+namespace Tests;
+
 use CodeIgniter\CLI\CLI;
 use CodeIgniter\Test\CIUnitTestCase;
 use CodeIgniter\Test\Filters\CITestStreamFilter;