Fix a bug when setting the default mock namespace

Autoloaders receive a classname without a leading `\`, which is totally expected.
By default, the [mock namespace pattern](https://github.com/atoum/atoum/blob/5ad65f9d2fd080e2600148ee8c361e3e56869256/classes/test.php#L1049-L1058) is `/^mock\\/i`, as expected.
However, when we do:
```php
$test->getMockGenerator()->setDefaultNamespace('mock');
```
we could expect the same behavior, but no. The mock namespace pattern is set to `/^\\mock\\/i`, which is going to reject all classnames in the mock autoloader.

This PR fixes the `setDefaultNamespace` method, where the bug is located, because a leading `\` is added.