Fix running tests for autoloaded classes

Should fix this issue: https://github.com/sebastianbergmann/phpunit/issues/529

This will always perform the check for $suiteClassName inside loaded classes
I think you should remove classes that have been loaded from `$untestedClasses`, i.e. something like

```php
foreach ($this->untestedClasses as $i => $className) {
    if (substr(...)) {
        ...
        unset($this->untestedClasses[$i]);
    }
}
```