Fix a race condition problem

I've fixed #3857.

And I am sorry. I didn't know my commit was notified to upstream automatically. 
I shouldn't have done force-push.
Is this not the latest source code ?

https://github.com/cakephp/cakephp/blob/master/lib/Cake/Test/Case/Model/Datasource/DboSourceTest.php

Certinly, missing function doc comment at line 36.