Fixes skipping of parent classes

Should fix this issue: #529

This will always perform the check for $suiteClassName inside loaded classes
Currently, newly discovered classes are always appended at the end of the array. Down below however, you start the traversal of the array from the beginning (foreach) although the actual class is likely to be at the end. If the array grows, this might cause a performance drop.

I'd make sure that the array indices are contiguous (array_merge() doesn't do that) and iterate the array from the back:

```php
foreach (array_diff(get_declared_classes(), $classes) as $class) {
    $this->untestedClasses[] = $class;
}

...

for ($i = count($this->untestedClasses) - 1; $i >= 0; --$i) {
    if (substr(...
}