However, the manager who interviewed me implied that the test is just a formality. I don't remember his exact words but he said something on the lines that the test is not that big of a deal and if you do verbally accept the offer, then you will get the job.
There's two ways he can have meant that - either that they view the test as generally being easy and people rarely fail it, or that he doesn't particularly care about the results of the test and having you take it is more in the nature of a box-ticking exercise than anything else. Obviously we can't know for sure what they were thinking, but unless you and the potential employer have wildly different ideas about the expected skill level for this role I'd be inclined to think the latter is more likely.
What are the chances that they rescind the offer based on my poor performance in the test? Can they do this after the verbal job agreement?
Verbal employment contracts are binding in Finland (well, as much any he-said she-said ever is I suppose) - but the conditional nature of the offer was pretty clear. I'm not completely au fait with Finnish rules around this but I'd be surprised if conditional offers weren't a thing.
I just gave the assesment test and i think i did poorly. It was much harder than i expected.
The thing with assessments given before a job is that unlike tests and exams you might encounter in an academic setting you often have no real gauge of what "doing well" means - and that can mean you think you did horribly when the reverse is true. I've had ones where I've felt like a complete failure for not finishing only to subsequently discover that no-one is expected to finish and I've actually gotten further than most! So really it doesn't matter how you think you did compared to your own expectations it only matters how you did compared to their expectations - and even then that only applies where they actually care about how well you did. Which in this case it sounds as though they might not really give it that much weight.