There is 'way too much CSL code in that test than necessary, which is distracting. It’s been a long time since I worked on sorting, and I had to dig into it myself to figure out what was going on. So … yeah, a comment would have been helpful. The result is correct, though.
Cite grouping is applied only if the citations are sorted in some way, otherwise they should be rendered in entry order. If you add this to the test, the seminal works of Aalto will be grouped:
<sort>
<key variable="author"/>
</sort>
(Edit: Cite grouping isn’t actually exercised in this one, since each author’s refs are all from the same year.)