Added tests for the GroupedSubjectIterator

This will avoid regressions for the fix done in #430. I haven't worked on tests this weekend, and so you merged before they got added.

It also adds an ArraySubjectIterator in Testwork for cases where the subjects cannot be lazy-loaded. 
I wanted a real iterator for the integration tests (mocking the iterator to be sure it worked well would have been a real pain), so adding this class made things easier, and I consider it can be part of the code itself for cases where lazyness is not an option. However, a lazy iterator should be always preferred whenever possible to limit memory usage.