Restore plugin unload.

tests/TestCase/Auth/FormAuthenticateTest.php

@@ -298,6 +298,7 @@ class FormAuthenticateTest extends TestCase
             'updated' => new Time('2007-03-17 01:18:31')
         ];
         $this->assertEquals($expected, $result);
+        Plugin::getCollection()->clear();
     }
 
     /**