MjaM:
The assessment you were asked to attend is basically like an interview. Most of the discussion will be aviation related but you are not required to have advanced knowledge of aviation or ATC (that's not the point of the assessment). It's about English language so they will be introducing some topics and all you have to do is discuss so they can check your English.
Have a look at the ICAO language proficiency requirements so you are familiar with the 6 categories being assessed and the requirement for each of the 6 levels (there's a table
here). Don't bother much with all the practice tests and sample tests and aviation english school webpages that will come up when you use a search engine. Those are mostly for pilots and are different to what you will do. They can be more confusing than helpful.
Crazy Voyager:
Indeed you will do the formal language proficiency test during training together with everyone else (the native speakers). But what you were told doesn't necessarily mean you are excluded from the recruitment assessment unless you were specifically told so. As MjaM found out it's a rather nasty surprise when shortly before starting you wait for the contract paperwork and instead you are told "you have to do one more test". So if you are at a similar stage in the recruitment and a non-native speaker check with HR so you know exactly what to expect.