After upgrading to PHPUnit v > 3.5 (I forgot) I kept on getting this error because I imported PHPUnit/Extensions/Database/TestCase.php:
require_once(...): failed to open stream: No such file or directory
I peeked at the PHPUnit folder in my /usr/share/php folder only to find out it isn’t there anymore.
In fact a look at the official source code reveals that it seems it has been removed.
So for now I am using version 3.4.15. But it’s not ideal as assertions on the table are still not supported. For now I am only able to use the CLEAN_INSERT feature which populates my database.
I solved this by installing db unit manually. I downloaded it from here. Then I replaced the incomplete Extensions/Database folder with the one from the download.