Fix Psr0Test to not print warning

Currently the Psr0Test prints a warning like this:

```
Configuration read from /.../phpunit.xml.dist

...............................................................  63 / 149 ( 42%)
.......................................................! The namespace LeadingSpace in /.../vendor/fabpot/php-cs-fixer/Symfony/CS/Fixer/Psr0Fixer.php does not match the file path according to PSR-0 rules
........ 126 / 149 ( 84%)
.......................

Time: 266 ms, Memory: 7.00Mb
```

Surrounded the test with outputbuffer, like in the other tests.