Set default namespace in atoum\phpMocker

Since we unlink a test suite name from a SUT name, the [`atoum\phpMocker` class](https://github.com/atoum/atoum/blob/a9503a7a0c5b1f5893f9ec01c35082dc4a6c4cd3/classes/php/mocker.php) has no default namespace and is not able to work properly. This patch fix this.

In other words, it restores the `$this->function->…` function mocking system :-).